Reconsidering Race Adjustment in Prenatal Alpha-Fetoprotein Screening

OBJECTIVE

Black racial designation is the only race for which adjustment is recommended for maternal prenatal serum alpha-fetoprotein (AFP) screening. The objective of this study is to reevaluate the relationship between maternal race and maternal serum AFP values in prenatal analyte screening.

METHODS

This was a single-center retrospective analysis of patients who underwent prenatal analyte screening between January 2007 and December 2020. Nomograms for raw maternal serum AFP values by gestational age were created and compared between patients identified as "Black" and "non-Black" on the laboratory requisition. Multivariable linear regression models were created to evaluate the relationship among gestational age, maternal weight, and maternal race on maternal serum AFP levels. The new models were compared with the laboratory-derived calculations, which used historically determined race adjustments.

RESULTS

A total of 43,997 patients underwent analyte screening, and 27,710 patients had complete data for analysis. Of these, 6% were identified as Black. Black patients had laboratory blood draws at a mean gestational age of 123 days, compared with 120 days in non-Black patients ( P <.001), and had higher maternal weight (mean 170 vs 161 lbs, P <.001). Nomograms for raw maternal serum AFP values did not differ between Black and non-Black patients ( P =.065). When adjusted for gestational age and maternal weight, no difference in maternal serum AFP values was identified between Black and non-Black individuals ( P =.81).

CONCLUSION

No difference in maternal serum AFP values was identified between Black and non-Black pregnant individuals when adjusted by maternal weight and gestational age at blood draw. These findings suggest that routine race-based adjustment of maternal serum AFP screening should be discontinued.

Prenatal screening for open neural tube defects

The era of prenatal screening for serious birth defects began in the 1970s with the discovery that amniotic fluid and maternal serum levels of alpha-fetoprotein (AFP) were increased in pregnancies affected by fetal open neural tube defects. Since then, prenatal screening has become a part of routine obstetric care. In this article, the use of AFP in prenatal screening for open neural tube defects is discussed in the context of the laboratory and the laboratory's interactions with the practicing obstetrician.

Maternal serum alpha-fetoprotein and altitude

Maternal serum alpha-fetoprotein (MS-alphaFP) testing is widely used to screen for fetal defects. MS-alphaFP concentrations are affected by a number of variables such as gestational age, maternal weight, number of fetuses, race, and insulin-dependent diabetes. Undefined geographic factors may also influence MS-alphaFP. We have examined the effect of altitude in a sample of 1063 MS-alphaFP results selected to span a range of altitudes. The study sample was subjected to linear regression with and without a term for altitude, and multiple-of-the-median (MoM) values were calculated before and after adjusting for altitude. The median MS-alphaFP was found to decrease an average of 1 ng/mL for every 1100 ft increase in altitude, a change approximately equivalent to that seen with an increase in maternal weight of 6 lb. Adjusting for altitude resulted in the reclassification of 36 of 1063 patient results (3.4%), although the clinical utility of this adjustment remains unexamined.

Evaluation of different weight correction methods for antenatal serum screening using data from two multi-centre programmes

Weight correction of serum markers is widely used when screening for Down's syndrome and open neural tube defects (NTD) because marker concentrations decrease with increasing maternal weight. Log-linear regression is frequently used for determining weight correction factors, but recently reciprocal-linear regression has been suggested to have advantages. We compared both methods of weight correction using data from two screening programmes carried out by this laboratory, one using alpha-fetoprotein (AFP) and total human chorionic gonadotrophin (HCG) (n = 129,143) and the other, AFP and free beta-HCG (n = 39,982). The reciprocal-linear method fitted the data more closely but did not significantly alter the detection rate or screen positive rate (SPR) for Down's syndrome or NTD with either dataset. Without correction, women heavier or lighter than average weight had significantly different SPRs for Down's syndrome and NTD compared with those weighing close to the median weight. Both correction methods smoothed out the variability in the SPR for Down's syndrome to a similar degree, but reciprocal-linear regression was much better at reducing the variability in SPR for NTD and its use is therefore worthwhile.

Influence of smoking habits on Down's syndrome risk evaluation at mid-trimester through biochemical screening

Expectant mothers who smoke have higher levels of maternal serum alpha-fetoprotein and lower levels of unconjugated estriol and total human chorionic gonadotrophin than non-smoking mothers. This significantly affects performance of screening for Down's syndrome. This study includes 22,169 pregnant women: 18,876 non-smokers, 2,660 smoking < or = 10 cigarettes/day, and 633 smoking > 10 cigarettes/day. Mean maternal age (32.6 years), maternal weight (60.5 kg), and gestational age (114.7 days) were similar or only slightly different between the three groups. To verify the effects of smoking on screening, we studied retrospectively 130 sequential Down's syndrome cases (47 from the screening program, 83 from the prenatal diagnosis program). The proportion of smokers in the Down's syndrome and unaffected pregnancies was similar, whilst the false-positive rate and detection rate, based on fetal outcome, differed: false-positive rates were 5.63% in smokers and 9.42% in non-smokers, and detection rate 55.6% in smokers and 83.0% in non-smokers. Since the prevalence of Down's syndrome pregnancies was the same at mid-trimester in smokers and non-smokers and the proportion of smokers was not related to maternal age, we propose an adjustment of the Down's syndrome risk evaluation algorithm according to smoking habits.

Perinatal outcomes of obese women: a review of the literature

OBJECTIVE

To review the literature addressing the effect of obesity on pregnancy outcomes and to identify practice and research implications.

DATA SOURCES

Computerized searches in Medline and CINAHL, as well as references cited in articles reviewed. Key words used in the search were as follows: pregnancy and obesity; pregnancy complications; weight gain and pregnancy; weight gain and complications; fat distribution and pregnancy and complications; and obstetrics and obesity.

STUDY SELECTION

Articles and comprehensive works from indexed journals in the English language relevant to key words and published after 1978 were evaluated.

DATA EXTRACTION

Data were extracted and organized under the following headings: methodologic issues; physiologic adjustments; antepartum, intrapartum, postpartum and newborn outcomes; and cost.

DATA SYNTHESIS

Obese pregnant women experience more gestational diabetes, neural tube defects, preeclampsia, induction, primary cesarean, and postpartum infection than pregnant women who are not obese.

CONCLUSIONS

Pregnant women who are obese are at increased risk for certain complications during pregnancy, birth, and postpartum. Little is known about the effect that fat distribution (upper versus lower, which is influential in nonpregnant populations) has on obstetric complications. Even less is known about obese pregnant women's perceptions of risk, changes in lifestyle, functioning, health behaviors, and symptoms experienced during pregnancy.

Repeat testing of mothers with high human chorionic gonadotrophin levels in Down's syndrome screening

Multiples of medians of serum markers are assumed to be independent of gestational age: every algorithm used for Down's syndrome risk evaluation is based on this hypothesis. However, our former observations suggested that multiples of medians of human chorionic gonadotrophin in Down's syndrome are dependent on gestatational age. Furthermore, observations on 84 Down's syndrome cases confirmed that human chorionic gonadotrophin multiples of medians in samples drawn at 15-17 weeks are approximately 10% lower than in samples drawn at 18-21 weeks, thus showing that the human chorionic gonadotrophin concentration decreases about 10% less than expected. The control group comprised 554 women with two blood samples and normal human chorionic gonadotrophin at first sampling. A further group of 532 women with multiples of medians at first sampling > 1.8 was examined with the aim of excluding an association between the human chorionic gonadotrophin trend in Down's syndrome and high starting values. The trend is peculiar to human chorionic gonadotrophin in Down's syndrome pregnancies and may help to explain the increase in detection rate with gestational age. Based on these findings, screening can be optimized, thus improving performance.

Antenatal screening for Down's syndrome

BACKGROUND

In 1968 the first antenatal diagnosis of Down's syndrome was made and screening on the basis of selecting women of advanced maternal age for amniocentesis was gradually introduced into medical practice. In 1983 it was shown that low maternal serum alpha fetoprotein (AFP) was associated with Down's syndrome. Later, raised maternal serum human chorionic gonadotrophin (hCG), and low unconjugated oestriol (uE3) were found to be markers of Down's syndrome. In 1988 the three biochemical markers were used together with maternal age as a method of screening, and this has been widely adopted. PRINCIPLES OF ANTENATAL SCREENING FOR DOWN'S SYNDROME: Methods of screening need to be fully evaluated before being introduced into routine clinical practice. This included choosing markers for which there is sufficient scientific evidence of efficacy, quantifying performance in terms of detection and false positive rates, and establishing methods of monitoring performance. Screening needs to be provided as an integrated service, coordinating and managing the separate aspects of the screening process. SERUM MARKERS AT 15-22 WEEKS OF PREGNANCY: A large number of serum markers have been found to be associated with Down's syndrome between 15 and 22 weeks of pregnancy. The principal markers are AFP, hCG or its individual subunits (free alpha- and free beta-hCG), uE3, and inhibin A. Screening performance varies according to the choice of markers used and whether ultrasound is used to estimate gestational age (table 1). When an ultrasound scan is used to estimate gestational age the detection rate for a 5% false positive rate is estimated to be 59% using the double test (AFP and hCG), 69% using the triple test (AFP, hCG, uE3), and 76% using the quadruple test (AFP, hCG, uE3, inhibin A), all in combination with maternal age. Other factors that can usefully be taken into account in screening are maternal weight, the presence of insulin dependent diabetes mellitus, multiple pregnancy, ethnic origin, previous Down's syndrome pregnancy, and whether the test is the first one in a pregnancy or a repeat. Factors such as parity and smoking are associated with one or more of the serum markers, but the effect is too small to justify adjusting for these factors in interpreting a screening test.

URINARY MARKERS AND FETAL CELLS IN MATERNAL BLOOD

Urinary beta-core hCG has been investigated in a number of studies and shown to be raised in pregnancies with Down's syndrome. This area is currently the subject of active research and the use of urine in future screening programmes may be a practical possibility. Other urinary markers, such as total oestriol and free beta-hCG may also be of value. Fetal cells can be identified in the maternal circulation and techniques such as fluorescent in situ hybridisation can be used to identify aneuploidies, including Down's syndrome and trisomy 18. This approach may, in the future, be of value in screening or diagnosis. Currently, the techniques available do not have the performance, simplicity, or economy needed to replace existing methods.

DEMONSTRATION PROJECTS

Demonstration projects are valuable in determining the feasibility of screening and in refining the practical application of screening. They are of less value in determining the performance of different screening methods. Several demonstration projects have been conducted using the triple and double tests. In general, the uptake of screening was about 80%. The screen positive rates were about 5-6%. About 80% of women with positive screening results had an invasive diagnostic test, and of those found to have a pregnancy with Down's syndrome, about 90% chose to have a termination of pregnancy. ULTRASOUND MARKERS AT 15-22 WEEKS OF PREGNANCY: There are a number of ultrasound markers of Down's syndrome at 15-22 weeks, including nuchal fold thickness, cardiac abnormalities, duodenal atresia, femur length, humerus length, pyelectasis, and hyperechogenic bowel. (ABSTRA

Alpha 1-fetoprotein (AFP) reference values in infants up to 2 years of age

The aim of this study was to establish reference values and factors associated with serum AFP elevation in infants. Five hundred twenty-four samples collected from infants up to the age of 2 years at the University Hospital Düsseldorf (Germany) were analyzed. At birth mean serum AFP levels were 41,687 ng/ml in 256 term babies and 158,125 ng/ml in 90 premature babies born before the 37th gestational week, excluding samples from children with factors known to be associated with AFP elevation. In the first 4 weeks of life, AFP levels decreased by 50% in 5.1 days in term babies. Between day 180 and 720 of life, AFP levels up to 87 ng/ml were within the 95.5% interval (assumed logarithmic normal distribution) with a mean of 8 ng/ml without a further decline. By the age of 2 years the infants of this study had not reached adult serum AFP levels (0-6 ng/ml).

Effect of parity correction on Down syndrome detection by the multiple-marker screening test

OBJECTIVE

Our purpose was to determine whether maternal parity affects analyte levels in the multiple-marker screening test for Down syndrome and to derive a correction factor and determine its effect on Down syndrome detection and screen-positive rates.

STUDY DESIGN

Our database consisted of 3039 multiple-marker screening test results and corresponding fetal karyotypes (2983 euploid and 56 Down syndrome). Cases were grouped by maternal parity as follows: 0 (n = 848), 1 (n = 1140), or > or = 2 (n = 1051). The mean multiple of the median of maternal serum alpha-fetoprotein, estriol, and human chorionic gonadotropin was determined for each group. A correction factor was derived for each parity group and applied to the database. Parity-corrected Down syndrome detection rates and screen-positive rates were determined.

RESULTS

Parity significantly affected the mean multiple of the median of human chorionic gonadotropin levels (p = 0.0001) but did not affect the values for estriol or maternal serum alpha-fetoprotein. Application of a parity correction factor for human chorionic gonadotropin increased the Down syndrome detection rate in women who had two or more pregnancies from 71% to 82% without increasing the overall screen-positive rate.

CONCLUSION

Human chorionic gonadotropin levels are significantly lower in multiparous women. Correcting human chorionic gonadotropin for maternal parity increases Down syndrome detection for women who had two or more pregnancies without affecting the overall screen-positive rate.

The influence of maternal weight on human chorionic gonadotropin in the multiple-marker screening test for fetal Down syndrome

OBJECTIVE

Our purpose was to determine the effect of maternal weight on human chorionic gonadotropin concentration in the multiple-marker screening test for fetal Down syndrome.

STUDY DESIGN

Two genetics databases were used: database I contained the results of 8297 multiple-marker screening tests and database II contained the results of 1936 multiple-marker screening tests and fetal karyotypes.

RESULTS

The overall screen-positive rate in database I was 7.1%; it was 7.5% in patients weighing < 180 pounds and 5.1% in patients weighing > or = 180 pounds (p = 0.001). Weight significantly affected the screen-positive rate only in women > or = 30 years old (p = 0.003 for 30 to 34 years, p = 0.00004 for > or = 35 years). A weight correction formula was derived; when applied to database II it eliminated individual weight-related differences but had no effect on the overall screen-positive rate or Down syndrome detection rate.

CONCLUSIONS

Human chorionic gonadotropin concentration is affected by maternal weight. A weight correction formula eliminates individual weight-related differences in the screen-positive rate but has no discernible effect on the overall screen-positive or Down syndrome detection rates.

Laboratory screening for genetic disorders and birth defects

Screening for inherited disease is a preventative health measure that started in the 1960s with the development of programs for the detection of PKU in newborns and that has had a major impact on reducing the burden of disease. Developments in technology have led to the availability of large scale testing for an increasing number of both acquired and genetic disorders. Laboratory testing is only one facet of a screening program and consideration should be given to availability of testing to all individuals, education regarding the program, effectiveness of treatment, long-term benefits both for individuals and society, ethical issues, and cost benefits. In this review, newborn, prenatal, and heterozygote screening are discussed.

Maternal serum alpha-fetoprotein screening: further consideration of low-volume testing

Unrecognized assay drift that may occur during low-volume (fewer than 500 specimens per week) maternal serum alpha-fetoprotein testing could result in either underestimation or overestimation of the number of pregnant women who are at increased risk of fetal malformations and genetic anomalies. Quality control software programs that incorporate the use of a multirule Shewhart chart are designed to detect assay drift. Careful selection of quality control sera for inclusion in analytic assays and appropriate application of a multirule quality control procedure to values that are obtained on these control materials should detect assay drift, regardless of the volume of patients' specimens in the run.

The effect of adjusting maternal serum alpha-fetoprotein levels for maternal weight in pregnancies with fetal open spina bifida. A United States collaborative study

The effect of adjustment of maternal serum alpha-fetoprotein levels for maternal weight was evaluated in 143 pregnancies associated with fetal open spina bifida. The cases were analyzed with gestational dates as initially assigned either by last menstrual period (73%) or by ultrasonography (27%). The weights of women with pregnancies affected by open spina bifida were found not to be different from unaffected pregnancies. Adjustment resulted in a net increase in detected cases at two commonly used cutoff points: 2.0 and 2.5 multiples of the median. At 2.0 multiples of the median, the detection rate increased from 80% to 84%, whereas at 2.5 multiples of the median the rate rose from 68% to 69%. Adjustment for maternal weight has previously been shown to reduce the false-positive rate in maternal serum alpha-fetoprotein screening for fetal open spina bifida by reducing variance in unaffected pregnancies; our data provide further support for consideration of maternal weight when interpreting material serum alpha-fetoprotein measurements.

Fetomaternal transfusion depends on amount of chorionic villi aspirated but not on method of chorionic villus sampling

Transcervical and transabdominal chorionic villus sampling are believed, on the basis of indirect evidence, to result in fetomaternal transfusion. We sought to measure this phenomenon by devising a simple method that would allow us to identify variables that influence fetomaternal transfusion. We investigated patients undergoing transcervical-chorionic villus sampling (n = 15) and transabdominal-chorionic villus sampling (n = 15), restricting the sample to subjects who required only a single catheter passage or needle insertion to obtain villi. Maternal serum alpha-fetoprotein was measured before and after the procedure along with alpha-fetoprotein concentration of the transport medium into which the villi had been aspirated. We first confirmed that the change in maternal serum alpha-fetoprotein levels after chorionic villus sampling, an indirect measure of fetomaternal transfusion, was indeed correlated with the alpha-fetoprotein concentration of transport medium into which the villi were aspirated (p = 0.0350). Fetomaternal transfusion next proved to be correlated with the amount of villi obtained (p = 0.0279). However, when adjusted for the amount of villi obtained, no significant difference was observed between transcervical and transabdominal-chorionic villus sampling with respect to the change in maternal serum alpha-fetoprotein levels after chorionic villus sampling (p = 0.8512). These data suggest that the magnitude of fetomaternal transfusion depends on the amount of villi obtained but not on the chorionic villus sampling method used.

Towards the prevention of neural tube defects

The value of maternal periconceptional vitamin supplementation in preventing the conception of fetuses with neural tube defects has yet to be assessed in a properly designed study. Avoidance of the birth of affected infants continues to rest on second-trimester maternal serum alphafetoprotein screening. The performance of screening programmes in achieving this objective is reviewed.

Prospective study of an inverse relationship between maternal glycosylated hemoglobin and serum alpha-fetoprotein concentrations in pregnant women with diabetes

A hypothesized inverse relationship between the concentration of glycosylated hemoglobin and serum alpha-fetoprotein was observed in a prospective study of 39 pregnant women with insulin-dependent diabetes as well as seven pregnant women with diabetes who did not require insulin (r = -0.434, p less than 0.002). No similar correlation was found among a selected population of healthy pregnant women (r = -0.129). Because the level of glycosylated hemoglobin in pregnancy correlates with poor outcome, including the occurrence of fetal anomaly, it may be important to quantify glycosylated hemoglobin in pregnancies with low alpha-fetoprotein levels. These results also suggest that the maternal concentration of glycosylated hemoglobin can be used to adjust serum alpha-fetoprotein values before their interpretation in the screening of pregnant women with diabetes.

Very low versus undetectable maternal serum alpha-fetoprotein values and fetal death

Very low maternal serum alpha-fetoprotein (MSAFP) levels (less than 10 ng/mL) are known to be associated with non-viable pregnancies, including conditions such as fetal death, molar pregnancies, and non-pregnancies. There has not been agreement, however, as to whether very low MSAFP levels indicate already existing fetal deaths or are actually predictive. We analysed 230 pregnancies with MSAFP levels less than 10 ng/mL from among 15,807 women (1.5 per cent) screened consecutively during a three-year period and identified 26 non-viable pregnancies, 22 of which were diagnosed sonographically as part of the screening process (17 missed abortions, 3 blighted ova, 2 non-pregnancies). Furthermore, 20 of these 22 pregnancies were associated with essentially undetectable MSAFP levels (less than 5 ng/mL). Our data indicate that pregnancies with MSAFP values less than 5 ng/mL are the group most strongly associated with fetal non-viability and that very low MSAFP values are not strongly predictive for fetal death.

Plasma alpha fetoprotein reference ranges in infancy: effect of prematurity

The dearth of plasma alpha fetoprotein reference ranges for preterm infants often impairs the clinical interpretation of plasma alpha fetoprotein data collected from ill babies. This study tested our hypothesis that meaningful plasma reference ranges could be established for preterm infants by a simple correction of patient age at sampling date for gestational age deficit at birth. Using a modified radioimmunoassay kit method, determinations of alpha fetoprotein were performed on capillary and venous blood samples collected from 56 babies aged from birth to 5 months with gestational ages ranging from 26 weeks to 43 weeks. Unmodified plasma alpha fetoprotein values were grouped according to patient age and examined statistically using established normal theory methods, but these yielded excessively wide reference intervals and non-Gaussian distribution parameters. Acceptable reference ranges were derived using logarithmic transformation of plasma alpha fetoprotein values and rearrangement against patient age corrected for gestational age deficit. These provisional reference ranges for plasma alpha fetoprotein in preterm (and term) infants are applied to groups of previously meaningless alpha fetoprotein results and used to test the potential usefulness of plasma alpha fetoprotein determination as a diagnostic marker in biliary atresia, hepatitis, and yolk sac derived tumours.

Low maternal serum alpha-fetoprotein and perinatal outcome

Pregnancy outcome was followed prospectively in women showing maternal serum alpha-fetoprotein values less than 0.4 multiple of the median. Using a radioimmunoassay later shown by others to produce a disproportionate number of low values, we nonetheless detected all three cases of autosomal trisomy (+18, +18, +21) at amniocentesis in 1531 women screened. Although two fetal losses and two autosomal trisomies (trisomy 18) occurred among a subgroup of only 15 women having two values less than 0.25 multiple of the median, fetal losses were in general far less frequent among the 99 women with at least one maternal serum alpha-fetoprotein value less than 0.4 multiple of the median than among women in previous reports. Comparing women with maternal serum alpha-fetoprotein values less than 0.4 multiple of the median against those with normal values (0.4 to 2.49 multiples of the median) also revealed no significant differences with respect to presence or absence of a variety of antepartum or intrapartum complications. Birth weight, gestational age, arterial cord pH, and Apgar scores also failed to differ significantly (one-way analysis of variance, p greater than 0.05). Women with a viable pregnancy who show low maternal serum alpha-fetoprotein values have a more favorable prognosis than previously claimed.

Maternal serum alpha-fetoprotein screening, maternal weight, and detection efficiency

Various methods of adjusting maternal serum alpha-fetoprotein levels based on maternal weight have been recommended in order to more appropriately assign risk in maternal serum alpha-fetoprotein screening. These adjustments may, however, result in a larger proportion of pregnancies designated as having increased risk and reduction in detection accuracy.

Critical issues in prenatal maternal serum alpha-fetoprotein screening for genetic anomalies

Prenatal screening, in this Clinical Opinion article, is defined as a population-based search for subgroups of pregnancy that by virtue of their maternal serum alpha-fetoprotein levels may be at increased risk for genetic anomalies. The identification of subgroups at risk for neural tube defects, ventral wall defects, and chromosomal trisomies presents clinicians with unprecedented information and issues, including the biomedical basis for prenatal screening, the distinction between screening and diagnosis, and the decision-making function of the patient. The interest and, perhaps, ambivalence of the clinical community to prenatal screening are understandable. Obstetricians ordering prenatal screens are challenged by medical data associated with epidemiology, pathology, and genetics, including: prevalence of disease, interpretive biochemical reporting, and the assessment and communication of patient-specific risks. Obstetricians ordering screens will be called on to provide information generally used by other specialists at other stages of care. What information, how it is biomedically determined, its form for communication to patients, and pitfalls in the process are discussed.

Screening for Down's syndrome using serum alpha fetoprotein: a retrospective study indicating caution

A report was made on the outcome of a four year retrospective study in 27 064 pregnancies, of the clinical efficiency, sensitivity, and specificity of a screening programme for Down's syndrome based on reported strategies related to the measurement of maternal serum alpha fetoprotein. This study identified 27 pregnancies affected by Down's syndrome with a median multiple of the median maternal serum alpha fetoprotein concentration of 0.82. This figure is considerably higher than that obtained from previous reports on this subject. With an age related multiple of the median maternal serum alpha fetoprotein strategy, 30.8% of Down's affected pregnancies were identified as well as 11.6% of unaffected pregnancies. Perhaps a United Kingdom collaborative study should begin to investigate the reasons for such wide population variance in the reports for the median multiple of the median for Down's affected pregnancies. Until such studies are carried out, screening for Down's syndrome based on low maternal serum alpha fetoprotein concentration is premature.

Does smoking influence serum alpha-fetoprotein levels in mid-trimester pregnancies?

The effect of cigarette smoking during pregnancy on serum alpha-fetoprotein (AFP) levels was studied in 1925 consecutive women at 16-18 weeks gestation who subsequently had liveborn singleton infants. Cotinine and AFP were measured in sera in all the women and answers to a smoking questionnaire were analysed in 894. Cotinine, a metabolic derivative of nicotine, serves as an objective measure of the average daily absorption of smoking products because of its relatively long half-life. Infants born to women who smoked were, on average, 198 g lighter than those born to non-smokers, even though maternal weights were comparable. Whether analysed by self-reporting or by serum cotinine, cigarette smoking had no measurable effect on serum AFP levels.

Maternal serum alpha-fetoprotein measurement: a screening test for Down syndrome

The median maternal serum alpha-fetoprotein (AFP) level at 14-20 weeks' gestation in 61 pregnancies associated with Down syndrome was 0.72 multiples of the median (MoM) value for a series of 36 652 singleton pregnancies unaffected by Down syndrome or neural-tube defect--a statistically significant reduction. The difference is great enough to form the basis of a screening test. By selecting for amniocentesis women with serum AFP levels less than or equal to 0.5 MoM at 14-20 weeks' gestation (excluding any of these that ultrasound cephalometry shows to have been due to the overestimation of gestational age) 21% of pregnancies with Down syndrome would be identified as well as 5% of unaffected pregnancies. If amniocentesis were offered to all women aged 38 years or more and, in addition, to younger women with serum AFP below specified maternal age-dependent cut-off levels (less than or equal to 1.0 MoM at 37 years, less than or equal to 0.9 at 36, less than or equal to 0.8 at 35, less than or equal to 0.7 at 34, less than or equal to 0.6 at 32-33, less than or equal to 0.5 at 25-31) 40% of pregnancies with Down syndrome and 6.8% unaffected pregnancies would be selected.

Clinical interpretation of maternal serum alpha-fetoprotein concentrations

Concentrations of maternal serum alpha-fetoprotein provide the basis for decisions to proceed to ultrasonography and amniocentesis in the multistaged screening/diagnostic process used for the prenatal detection of open neural tube defects, abdominal wall defects, and twins. The concentration of maternal serum alpha-fetoprotein at or above which women should be advised that amniocentesis is available (cutoff levels for amniocentesis) varies, depending upon a number of factors, such as maternal weight, race, residence, and gestational age. We briefly describe a methodology for computing the predicted risks of fetal conditions associated with a given concentration of maternal serum alpha-fetoprotein adjusted for important variables. This adjustment methodology provides a straightforward means for clinical laboratories to report results of assays of maternal serum alpha-fetoprotein in terms of predicted risks, to facilitate understanding by the physician and patient of the clinical meaning of the results of maternal serum alpha-fetoprotein testing.

Prevention of neural tube defects in an urban health district

An evaluation of voluntary prenatal screening for neural tube defects with serum alphafetoprotein (s-AFP) is presented. During a three year period, there were 52 fetuses with neural lesions. Of 46 mothers who bore a fetus with an open neural lesion, 25 were detected, of whom 23 agreed to the termination of their pregnancies. An unscreened group of 2331 mothers (17% of all deliveries) produced 14 fetuses with neural tube defects, an incidence of 6.0 per 1000, 1.8 times the incidence (3.3 per 1000) in the screened group. Pitfalls occurring in the assessment of hyper-alphafetoproteinemia included inaccurate gestational dating and allowance for excessive body weight. The results of a robust AFP-NTD screening programme supported in selected cases by ultrasonar visualisation of the spine argue for its continuance in this district.

Smoking habits and maternal serum alpha-fetoprotein levels during the second trimester of pregnancy

In a prospective study the influence of cigarette smoking on maternal serum alpha-fetoprotein levels at 16 weeks gestation was examined. Significantly higher levels were found in 120 smokers compared with 138 non-smokers (median 54.0 and 44.3 micrograms/1 respectively, P less than 0.001). No difference in maternal body weight between the two groups could account for the results. The possibility of smoking-induced increased permeability of the placental barrier is discussed.

Fetal sex-related differences in maternal serum alpha-fetoprotein during the second trimester of pregnancy

In an analysis of data gathered from routine maternal serum alpha-fetoprotein screening, we determined that the sex of the fetus is significantly correlated with differences in maternal serum alpha-fetoprotein concentration during the second trimester of pregnancy. Between the fifteenth and nineteenth gestational weeks, the mean maternal serum alpha-fetoprotein level is significantly higher for the male fetus than for the female fetus. There was no difference, however, in the rate of increase in maternal serum alpha-fetoprotein concentration with time between male and female fetuses during the weeks of gestation that were studied. No apparent cause for these observations has been demonstrated.

Maternal serum alpha-fetoprotein screening in North Carolina: experience with more than twelve thousand pregnancies

A total of 12,084 patients participated in a maternal serum alpha-fetoprotein (AFP) screening program in central North Carolina between July 1, 1978, and June 30, 1982. Fifteen open neural tube defects were detected and three others were missed, resulting in a detection rate of 83%. With a cutoff of 2.5 times the normal median, 3.7% of patients screened had a single maternal serum AFP elevation, 2.1% had two successive elevations, and 1.2% became candidates for amniocentesis. Of those patients offered amniocentesis, one in 10 was found to have a fetus with a neural tube defect. No normal fetuses have been aborted. Patients with maternal serum AFP elevations were shown to have a substantially increased risk of fetal loss. Low maternal serum AFP levels were also a significant finding and led to recognition of less advanced gestational age, fetal death, or molar pregnancy in a number of cases.

The effect of maternal weight on maternal serum alpha-fetoprotein levels

In 902 singleton pregnancies, maternal serum alpha-fetoprotein (AFP) levels between 15 and 20 weeks of pregnancy were significantly related to maternal weight (r = 0.24, p less than 0.0001). Lighter women had on average higher AFP levels than heavier women, perhaps on account of the greater concentration of AFP in their relatively smaller volume of blood. The mean AFP level for women weighing less than 45 kg was 68% higher than the mean level for women weighing 85 kg or more. Maternal weight was found to be an important factor which could account for false positive AFP results in antenatal screening for open neural tube defects. A policy of adjusting maternal serum AFP values according to maternal weight among women with borderline positive results could reduce the number having a diagnostic amniocentesis with only a negligible loss of detection for open spina bifida. With such a policy, using a cut-off level of 2.5 x normal median, the false positive rate in the 902 women screened would have been reduced fom 2.8% to 2.0%.