Performance of First-Trimester Screening between Gestational Weeks 7 and 13

Background: Screening for fetal chromosome abnormalities in the first trimester includes analysis of the serological markers pregnancy-associated plasma protein A (PAPP-A) and free β human choriogonadotropin (free β hCG). The blood sample is traditionally taken around week 12 of gestation, but the performance of earlier blood sampling is not well documented. Methods: We studied 44 537 singleton pregnancies. Complete first-trimester screening took place between November 2003 and March 2009, and blood samples were taken between 7 weeks + 5 days and 13 weeks + 6 days. Results: Of 120 cases of trisomy 21, 108 were diagnosed in the first-trimester screening (detection rate 90%). When the blood sample was taken before gestational week 10, the detection rate of trisomy 21 was 97% (70 of 72), whereas 80% were detected (38 of 48) after week 10 (χ2 = 0.0035). For trisomy 18, trisomy 13, and triploidy, 65% (13 of 20) were detected before gestational week 10, and 73% (11 of 15) after (not significant). All 6 cases of triploidy before and after gestational week 10 were detected. The screen positive rate and the maternal age were similar before and after week 10 of gestation. Conclusions: Screening for fetal aneuploidy can be performed with good results with the blood sample taken as early as week 7 of gestation. Blood samples taken before gestational week 10 showed a high detection rate of fetal trisomy 21, with no difference in the detection of fetal trisomy 18, trisomy 13, or triploidy.