First trimester maternal serum alpha-fetoprotein screening for Down syndrome and other aneuploidies

Screening for Down's syndrome: changes in marker levels and detection rates between first and second trimesters

OBJECTIVE

To monitor changes with gestation in levels of alpha-fetoprotein (AFP), free beta human chorionic gonadotrophin (F beta hCG) and pregnancy associated plasma protein-A (PAPP-A) in Down's syndrome pregnancies and to compare risks estimated in the first trimester with those obtained by routine screening in the second trimester for the same pregnancies.

DESIGN

In each of 47 Down's syndrome pregnancies two maternal serum samples were obtained, one in the first trimester and one in the second trimester. Comparison of marker levels with 10,600 first trimester controls and a smaller sample of second trimester controls allowed case identification criteria based on optimum marker combinations to be developed and compared directly between trimesters.

SETTING

Biochemical genetics laboratory.

RESULTS

F beta hCG was an effective marker of Down's syndrome in both the first and second trimesters. PAPP-A levels were significantly reduced in trisomy 21 pregnancies in the first trimester only. Using a population model, these two markers in combination with maternal age gave an overall detection rate of 55% for a 5% false positive rate in the first trimester. For the paired first and second trimester samples, three of six cases classified as low risk by routine second trimester screening were classified as high risk by the first trimester screening protocol of F beta hCG/PAPP-A/maternal age. However, fifteen cases identified as high risk by routine second trimester screening were classified as low risk in the first trimester, a net loss in detection of 12 cases by first trimester screening.

CONCLUSION

The data suggest that first trimester detection rates for Down's syndrome using a combination of F beta hCG and PAPP-A may vary with gestation and will be lower than those currently obtained by routine second trimester screening with AFP/hCG.

First trimester maternal serum alpha-fetoprotein in fetal trisomies

OBJECTIVE

To evaluate the potential value of maternal serum alpha-fetoprotein concentration in the detection of fetal trisomy at 10 to 13 weeks gestation and to examine the possible association between maternal serum alpha-fetoprotein and fetal nuchal translucency thickness.

DESIGN

Cross-sectional study.

SETTING

Harris Birthright Research Centre for Fetal Medicine, King's College Hospital Medical School, London.

SUBJECTS AND METHODS

Maternal serum alpha-fetoprotein concentration was measured at 10 to 13 weeks gestation in samples from 57 pregnancies with fetal trisomies (trisomy 21 (n = 35), trisomy 18 (n = 16), and trisomy 13 (n = 6)) in 228 matched controls in whom the fetal nuchal translucency was < 3 mm and in 114 chromosomally normal fetuses with translucency > or = 3 mm.

RESULTS

In the control group maternal serum alpha-fetoprotein increased significantly with fetal crown-rump length (r = 0.451). In this group, the median maternal serum alpha-fetoprotein was not significantly different from that in the groups with trisomy 21 (median = 0.84 MoM), trisomy 18 (median = 0.86 MoM), or trisomy 13 (median = 0.94 MoM), respectively. Neither in the control group nor in the group with trisomic fetuses was maternal serum alpha-fetoprotein significantly associated with fetal nuchal translucency thickness (r = 0.01 and r = 0.03).

CONCLUSION

Measurement of maternal serum alpha-fetoprotein concentration in the first trimester of pregnancy is not likely to be useful in the prediction of fetal trisomies.