Prospective evaluation of prenatal maternal serum screening for trisomy 18

Ectopia cordis in a fetus with trisomy 18

Trisomy 18 is the second-most common autosomal trisomy and represents one third of the chromosomal trisomies identified prenatally. We present a case of a fetus with trisomy 18 in which thoracoabdominal ectopia cordis was detected prenatally; it was noted as a dominant defect on a sonographic examination performed at 19 weeks' gestation. Furthermore, our case exhibits a rare type of multiple-marker screening result with increased maternal serum alpha-fetoprotein. This case demonstrates the need for thorough sonographic evaluation of the fetal thoracoabdominal wall as early as possible, because fetal ectopia cordis can be the major abnormality of trisomy 18.

Prenatal diagnosis for detection of aneuploidy: the options

The value of all noninvasive prenatal tests must be viewed with the perspective of the consequences of invasive testing. Regarding second trimester noninvasive testing, biochemical screening is more accurate in establishing risk than maternal age alone. One or more major ultrasound abnormalities, nuchal thickening, or a shortened humerus should raise concern for Down syndrome regardless of the patient's a priori risk based on age or biochemical markers. Isolated minor ultrasound markers should not be used in calculating risk in low-risk patients regarding Down syndrome unless the biochemical profile already places the patient at risk or in a borderline risk zone. If the ultrasound finding is hyperechoic bowel, problems other than aneuploidy may be the cause, including cystic fibrosis, infection, or hemorrhage, and these problems must be considered if hyperechoic bowel is an isolated finding. Improved risk adjustment seems to be applicable to a priori high-risk patients with completely normal sonograms. Genetic sonograms with specific risk adjustment schemata may be used to adjust a priori risk (either maternal age or biochemical screening results) at centers in which this has proven to be accurate, but whether this is statistically sound remains to be determined. The goal of second trimester ultrasound screening is to identify at-risk fetuses better and offer invasive testing to a more select group of patients. As the value of first trimester screening becomes more evident and practical, and if the risk of chorionic villus sampling becomes an acceptable norm, the patient population that reaches the second trimester of pregnancy will be select. Therefore, we can anticipate that second trimester screening and invasive testing may be needed only in a minority of cases, and the practice standards of prenatal testing and sonography (including minor ultrasound markers) will change entirely.

Maternal serum screening for fetal trisomy 18: benefits of patient-specific risk protocol

OBJECTIVE

Our goal was to evaluate the effectiveness of two approaches to screen pregnancies for trisomy 18.

STUDY DESIGN

We analyzed the outcome of all pregnancies that were screen positive for trisomy 18 by multiple marker screening (alpha-fetoprotein, unconjugated estriol, and human chorionic gonadotropin) from May 1993 to June 1998. We compared the results of a fixed cutoff protocol to a protocol that incorporates maternal age to generate a patient-specific risk figure.

RESULTS

A total of 45,145 patients were screened. By using the fixed cutoff protocol, 113 patients (0.25%) were screen positive. The risk-based approach was associated with a 0.55% screen-positive rate (250 patients). Eight of 12 cases (67% detection rate) of trisomy 18 were identified by using the risk method, and only 5 cases (42% detection rate) were detected by using the fixed cutoff method. By using the risk-based protocol, 21 pregnancies with chromosomal abnormalities (8, trisomy 18; 7, triploidy; 5, trisomy 21; and 1, mosaic 45X/46XX) were detected. Subsequent fetal death occurred for 42 patients whose fetuses were chromosomally normal and without structural malformations.

CONCLUSION

The patient-specific risk protocol to screen for trisomy 18 is a beneficial adjunct to screening programs already in place for Down syndrome and neural tube defects. Patients found to be screen positive for trisomy 18 are at significant risk for adverse pregnancy outcome.

Maternal serum screening for Down's syndrome, open neural tube defects and trisomy 18

Maternal serum screening identifies women at an increased risk of a pregnancy with Down's syndrome or trisomy 18 or an open neural tube defect. The triple test, consisting of maternal serum alpha-fetoprotein, unconjugated estriol and human chorionic gonadotropin was carried out by a chemiluminescence immunoassay method in our laboratory. The study consisted of 373 pregnant women. The gestational range for the study group was 14-22 weeks. The mean maternal age for the study group was 28.53 +/- 5.46 years (range 17.4 to 43.5 years); 9.1% of the women were considered at high risk for Down's syndrome based on the test results. In our study the detection rate for Down's syndrome by prenatal karyotyping was 66.6%. Maternal serum screening allows reduction of the number of women requiring amniocentesis without a significant decrease in the detection rate.

The evolution of the thyroid hormone distributor protein transthyretin in the order insectivora, class mammalia

Thyroid hormones are involved in the regulation of growth and metabolism in all vertebrates. Transthyretin is one of the extracellular proteins with high affinity for thyroid hormones which determine the partitioning of these hormones between extracellular compartments and intracellular lipids. During vertebrate evolution, both the tissue pattern of expression and the structure of the gene for transthyretin underwent characteristic changes. The purpose of this study was to characterize the position of Insectivora in the evolution of transthyretin in eutherians, a subclass of Mammalia. Transthyretin was identified by thyroxine binding and Western analysis in the blood of adult shrews, hedgehogs, and moles. Transthyretin is synthesized in the liver and secreted into the bloodstream, similar to the situation for other adult eutherians, birds, and diprotodont marsupials, but different from that for adult fish, amphibians, reptiles, monotremes, and Australian polyprotodont marsupials. For the characterization of the structure of the gene and the processing of mRNA for transthyretin, cDNA libraries were prepared from RNA from hedgehog and shrew livers, and full-length cDNA clones were isolated and sequenced. Sections of genomic DNA in the regions coding for the splice sites between exons 1 and 2 were synthesized by polymerase chain reaction and sequenced. The location of splicing was deduced from comparison of genomic with cDNA nucleotide sequences. Changes in the nucleotide sequence of the transthyretin gene during evolution are most pronounced in the region coding for the N-terminal region of the protein. Both the derived overall amino sequences and the N-terminal regions of the transthyretins in Insectivora were found to be very similar to those in other eutherians but differed from those found in marsupials, birds, reptiles, amphibians, and fish. Also, the pattern of transthyretin precursor mRNA splicing in Insectivora was more similar to that in other eutherians than to that in marsupials, reptiles, and birds. Thus, in contrast to the marsupials, with a different pattern of transthyretin gene expression in the evolutionarily "older" polyprotodonts compared with the evolutionarily "younger" diprotodonts, no separate lineages of transthyretin evolution could be identified in eutherians. We conclude that transthyretin gene expression in the liver of adult eutherians probably appeared before the branching of the lineages leading to modern eutherian species.

A retrospective evaluation of second-trimester serum screening for fetal trisomy 18: experience of two laboratories

A retrospective study on screening methods for fetal trisomy 18 has been carried out in two different laboratories using the serum parameters: total human chorionic gonadotropin (hCG), unconjugated oestriol (uE3), and alpha-fetoprotein (AFP) in different combinations and in single marker protocols. Laboratory A (L(A)) utilized a radio-immunoassay to examine 38 fetal trisomy 18 cases and laboratory B (L(B)) utilized an enzyme-immunoassay to examine 33 trisomy 18 cases. As unaffected references the whole routine cohorts of each laboratory were used (L(A): 29 043; L(B): 4264). In both trisomy 18 study groups the median hCG and uE3 multiples of the median (MoM) values were markedly declined (L(A): 0.21 MoM, 0.37 MoM; L(B): 0.31 MoM, 0.44 MoM). Even after exclusion of trisomy 18 cases with combined neural tube or ventral wall defects the medians of AFP MoM values were only moderately declined (L(A): 0.73 MoM; L(B): 0.8 MoM). Receiver-operator characteristic (ROC) curves after multivariate discriminance analysis and single marker evaluation demonstrated that the difference of efficiency between a combination of hCG, uE3 and AFP, and a combination of hCG and uE3 is small but that any of these combinations are more efficient than a combination of hCG and AFP or single marker protocols, respectively. At a risk cut-off generating a false-positive rate of one per cent the most effective marker combination detected 31 of 38 (81.6 per cent) affected pregnancies in L(A) and 25 of 33 (75.8 per cent) in L(B). The differences in sensitivity and specificity seem to be due to the different analytical systems being utilized by the two laboratories.