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 beta human choriogonadotropin (free beta 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 (chi(2) = 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.