Prenatal screening for chromosome abnormalities

Prenatal diagnosis: the irresistible rise of the 'visible fetus'

Prenatal diagnosis was developed in the 1970s, a result of a partly contingent coming together of three medical innovations-amniocentesis, the study of human chromosomes and obstetrical ultrasound-with a social innovation, the decriminalization of abortion. Initially this diagnostic approach was proposed only to women at high risk of fetal malformations. Later, however, the supervision of the fetus was extended to all pregnant women. The latter step was strongly favoured by professionals' aspiration to prevent the birth of children with Down syndrome, an inborn condition perceived as a source of suffering for families and a burden on public purse. Experts who promoted screening for 'Down risk' assumed that the majority of women who carry a Down fetus will decide to terminate the pregnancy, and will provide a private solution to a public health problem. The generalization of screening for Down risk increased in turn the frequency of diagnoses of other, confirmed or potential fetal pathologies, and of dilemmas linked with such diagnoses. Debates on such dilemmas are usually limited to professionals. The transformation of prenatal diagnosis into a routine medical technology was, to a great extent, an invisible revolution.

Options for Down Syndrome Screening: What Will Women Choose?

Down syndrome screening has been offered to pregnant women since the early 1980s. Protocols have changed as research confirmed improvements that result in higher detection rates and lower false-positive rates. Results from 2 clinical trials evaluating screening protocols that include ultrasound measurement of nuchal translucency and biochemical testing in the first and second trimester are now available. First-trimester screening is an option if there are adequate ultrasound, diagnostic, and counseling services available. Regional variation in the availability of these services may limit the implementation of first-trimester screening. Combining screening tests for Down syndrome from both trimesters as an integrated test offers the highest detection rate with the lowest false-positive rate. The possibility of avoiding a positive screen will make this an attractive option for some. Timing, detection rate, false-positive rates, and personal factors influence the decision women make regarding screening versus diagnostic testing. This article reviews the efficacy of current protocols for Down syndrome screening. Accurate information about available screening tests will facilitate informed decisions about screening and testing.

Decisional needs assessment regarding Down syndrome prenatal testing: a systematic review of the perceptions of women, their partners and health professionals

OBJECTIVE

To identify decisional needs of women, their partners and health professionals regarding prenatal testing for Down syndrome through a systematic review.

METHODS

Articles reporting original data from real clinical situations on sources of difficulty and/or ease in making decisions regarding prenatal testing for Down syndrome were selected. Data were extracted using a taxonomy adapted from the Ottawa Decision-Support Framework and the quality of the studies was assessed using Qualsyst validated tools.

RESULTS

In all 40 publications covering 32 unique studies were included. The majority concerned women. The most often reported sources of difficulty for decision-making in women were pressure from others, emotions and lack of information; in partners, emotion; in health professionals, lack of information, length of consultation, and personal values. The most important sources of ease were, in women, personal values, understanding and confidence in the medical system; in partners, personal values, information from external sources, and income; in health professionals, peer support and scientific meetings.

CONCLUSION

Interventions regarding a decision about prenatal testing for Down syndrome should address many decisional needs, which may indeed vary among the parties involved, whether women, their partners or health professionals. Very little is known about the decisional needs of partners and health professionals.

A high oocyte yield for intracytoplasmic sperm injection treatment is associated with an increased chromosome error rate

OBJECTIVE

To compare the chromosome error rate among oocytes from stimulated ovaries after retrieval of 1-5 oocytes, 6-10 oocytes, and >10 oocytes.

DESIGN

Retrospective cohort study.

SETTING

A university-based human genetic institute in collaboration with a private fertility center.

PATIENT(S)

Nine hundred thirty-three women undergoing intracytoplasmic sperm injection (ICSI) with a poor prognosis.

INTERVENTION(S)

Oocyte collection with ovarian stimulation. Polar body testing of ICSI oocytes for common chromosome errors.

MAIN OUTCOME MEASURE(S)

Chromosome error rate in oocytes, as determined by five-color fluorescence in situ hybridization.

RESULT(S)

In women less than 35 years and women between 35 and 40 years undergoing the first ICSI cycle, oocytes from the high-yield group had an increased likelihood for detectable chromosome errors (50.9% and 54.6%, respectively), compared to the intermediate-yield group (34.9% and 43.8%) and the low-yield group (23.3% and 41.2%). The overall high rate (>or=50%) of chromosomally abnormal oocytes in women more than 40 years appeared to be mainly due to the maternal age effect and increased only slightly with oocyte yield.

CONCLUSION(S)

Oocyte yield may be considered as an indicator of ovarian response to hormone stimulation. In women up to 40 years a high yield of oocytes after superovulation is associated with an increased chromosome error rate.

Normal obstetric ultrasound reduces the risk of Down syndrome in fetuses of older mothers

The objective of this study is to determine whether a normal fetal morphology ultrasound scan in women older than 35 years reduces the risk of aneuploidy. We reviewed the results of amniocentesis and second trimester sonogram in all women older than 35 years from 1991 to 1995. None had prior screening. We excluded fetuses with structural anomalies. We determined the sensitivity and specificity of minor markers in detecting Down syndrome and also determined the reduction in risk of a normal sonogram. Among the 2060 women older than 35 years giving birth during the study period, 16 (0.78%) delivered an infant with Down syndrome. Of the 16 fetuses, two had no prenatal testing or ultrasound, two had invasive testing but no second trimester sonogram, five had a normal sonogram and seven had one or more sonographic markers of Down syndrome. At least 17% of women older than 35 years did not participate in prenatal testing or ultrasound. Ultrasound detected Down syndrome with a sensitivity of 59% (95% confidence interval: 45-72%), a false-positive rate of 10.6% (9.4-11.8%) and a positive predictor value of 1 in 9. The likelihood of having normal karyotype if the sonogram was normal was 0.46 (0.31-0.61). In women older than 35 years, a normal second trimester sonogram reduces the risk of Down syndrome by more than 50%. At least 17% of women older than 35 years do not participate in prenatal testing or ultrasound.

Methylation Reprogramming and Chromosomal Aneuploidy in In Vivo Fertilized and Cloned Rabbit Preimplantation Embryos1

Active demethylation of the paternal genome but not of the maternal genome occurs in fertilized mouse, rat, pig, and bovine zygotes. To study whether this early demethylation wave is important for embryonic development, we have analyzed the global methylation patterns of both in vivo-fertilized and cloned rabbit embryos. Anti-5-methylcytosine immunofluorescence of in vivo-fertilized rabbit embryos revealed that the equally high methylation levels of the paternal and maternal genomes are largely maintained from the zygote up to the 16-cell stage. The lack of detectable methylation changes in rabbit preimplantation embryos suggests that genome-wide demethylation is not an obligatory requirement for epigenetic reprogramming. The methylation patterns of embryos derived from fibroblast and cumulus cell nuclear transfer were similar to those of in vivo-fertilized rabbit embryos. Fluorescence in situ hybridization with chromosome-specific BACs demonstrated significantly increased chromosomal aneuploidy rates in cumulus cell nuclear transfer rabbit embryos and embryos derived from nuclear transfer of rabbit fibroblasts into bovine oocytes compared with in vivo-fertilized rabbit embryos. The incidence of chromosomal abnormalities was correlated with subsequent developmental failure. We propose that postzygotic mitotic errors are one important explanation of why mammalian cloning often fails.

Prospective evaluation of a first trimester screening program for Down syndrome and other chromosomal abnormalities using maternal age, nuchal translucency and biochemistry in an Australian population

BACKGROUND

A combination of maternal age and ultrasound assessment of the nuchal translucency (NT) has been used in the first trimester to screen for chromosomal abnormality. In the United Kingdom, the addition of NT screening was shown to be beneficial.

AIMS

To report the sensitivity of combined first trimester biochemistry and ultrasound screening for Down syndrome in an Australian private practice specialising in obstetric ultrasound.

METHODS

A prospective study in a private obstetric ultrasound practice. Over 22 months, 2121 patients were screened and data was analysed for sensitivity (detection) and false positive rates for all chromosome abnormalities.

RESULTS

There were 17 chromosomal abnormalities, five of which were Down syndrome. Using maternal age alone or age and biochemistry, four of the Down syndrome cases were detected for a 29 and 19% false positive rate, respectively. Using age and NT or age, NT and biochemistry, all the Down syndrome cases were detected, for a false positive rate of 5.7 and 7.2%, respectively. The difference in detection rates for Down syndrome or other chromosomal abnormalities, using the four screening methods, did not reach statistical significance. However, the false positive rates in screening methods without ultrasound to assess the NT was significantly higher (P < 0.01).

CONCLUSIONS

A combination of maternal age, NT and maternal serum biochemistry gives a high detection rate for both trisomy 21 and other chromosomal abnormalities. Down syndrome screening using either maternal age alone or age in combination with first trimester biochemistry conferred screen positive rates significantly higher than when combined with NT.

An outcomes analysis of five prenatal screening strategies for trisomy 21 in women younger than 35 years

OBJECTIVE

This study was undertaken to examine the cost-effectiveness and procedural-related losses associated with 5 prenatal screening strategies for fetal aneuploidy in women under 35 years old.

STUDY DESIGN

Five prenatal screening strategies were compared in a decision analysis model: triple screen: maternal age and midtrimester serum alpha-fetoprotein, human chorionic gonadotropin (hCG), and unconjugated estriol; quad screen: triple screen plus serum dimeric inhibin A; first-trimester screen: maternal age, serum pregnancy-associated plasma protein A and free beta-hCG and fetal nuchal translucency at 10 to 14 weeks' gestation; integrated screen: first-trimester screen plus quad screen, but first-trimester results are withheld until the quad screen is completed when a composite result is provided; sequential screen: first-trimester screen plus quad screen, but the first-trimester screen results are provided immediately and prenatal diagnosis offered if positive; later prenatal diagnosis is available if the quad screen is positive. Model estimates were literature derived, and cost estimates also included local sources. The 5 strategies were compared for cost, the numbers of Down syndrome fetuses detected and live births averted, and the number of procedure-related euploid losses. Sensitivity analyses were performed for parameters with imprecise point estimates.

RESULTS

In the baseline analysis, sequential screening was the least expensive strategy ($455 million). It detected the most Down syndrome fetuses (n=1213), averted the most Down syndrome live births (n=678), but led to the highest number of procedure-related euploid losses (n=859). The integrated screen had the fewest euploid losses (n=62) and averted the second most Down syndrome live births (n=520). If fewer than 70% of women diagnosed with fetal Down syndrome elect to abort, the quad screen became the least expensive strategy.

CONCLUSION

Although sequential screening was the most cost-effective prenatal screening strategy for fetal trisomy 21, it had the highest procedure-related euploid loss rate. The patient's perspective on detection versus fetal safety may help define the optimal screening strategy.

Prenatal diagnosis for chromosome abnormalities: past, present and future

Prenatal diagnosis for chromosome abnormalities has been available for over 30 years. The most common referral indication is a raised risk of Down's syndrome, and diagnosis has, until recently, been carried out by culture of cells from invasive prenatal sampling, followed by full karyotype analysis, with a waiting time of around 2 weeks for results. More recent developments in fluorescence in situ hybridisation (FISH) and quantitative fluorescence-PCR techniques have led to rapid 1-2 d reporting for Down's syndrome, opening the way to the possibility of targeted testing based on referral indication, thus reducing the incidence of difficult counselling issues and potentially unnecessary pregnancy terminations following the unexpected discovery of anomalies such as balanced chromosome rearrangements. The future of prenatal diagnosis must lie in the non-invasive diagnosis of Down's syndrome using fetal cells from maternal circulation.

Factors affecting outcomes of prenatally-diagnosed tumours

OBJECTIVE

The outcomes of prenatally-diagnosed tumours affect obstetrical management and parental decisions. The present study reviews the factors affecting outcomes for fetuses with prenatally-diagnosed tumours.

METHODS

Medical records of all fetuses referred to our institutions with antenatally-diagnosed tumours were reviewed for the type and location of the tumours, results of treatment and/or causes of death.

RESULTS

From January 1994 to May 2001, there were 15 fetuses with antenatally- diagnosed tumours: mesoblastic nephroma (MN) (n=2); neuroblastoma (NB) (n=2); cystic hygroma (CH) (n=3); intracranial germ cell tumour (IGCT) (n=2); sacrococcygeal teratoma (SCT) (n=3) and haemangioma (liver, n=2; limb, n=1). One mother had termination of pregnancy for her fetal SCT. Three mothers had Caesarean section for large fetal heads (CH, n=2; IGCT, n=1). Three fetuses died; two with IGCT and one with SCT, who died of heart failure. Two newborns with CH needed emergency intubation and, later, one of them had tracheostomy. One baby had cardiac failure resulting from a lower limb haemangioma and needed drug therapy. All solid tumours (MN, NB, SCT) of the live births had no recurrence after surgery with or without adjuvant chemotherapy.

CONCLUSION

Prenatally-diagnosed tumours without any other associated abnormality cause morbidity and mortality because of their location and vascularity. Solid tumours are relatively benign.