CROWN-RUMP LENGTH IN ANEUPLOID FETUSES - IMPLICATIONS FOR FIRST-TRIMESTER BIOCHEMICAL SCREENING FOR ANEUPLOIDIES

This study examined the effect of estimation of gestational age from t he menstrual history compared with that from crown-rump length (CRL) m easurement on the detection rate of screening for aneuploidies in the first trimester. Pregnancy-associated plasma protein A (PAPP-A) was as sayed in blood collected prior to chorionic villus sampling in 356 wom en with unaffected pregnancies End 28 women with an aneuploid pregnanc y. There were 14 Down's syndrome (DS) pregnancies. All pregnancies wer e dated from menstrual history and CRL measurement. The average CRL ge station in the aneuploid population was 2.5 days less than that derive d from the LMP (95 per cent confidence interval (CI) for LMP-CRL gesta tion: using the algorithm based on unaffected pregnancies 0-3.5 days; using the matched case-control approach 1-4.5 days). The average CRL g estation in the DS population was 2 days less but this did not reach s tatistical significance (95 per cent CI for LMP-CRL gestation: using t he algorithm -1 to 4.5 days; using the matched case-control approach 0 to 5.5 days). The detection rate of aneuploidies in the first trimest er using maternal serum PAPP-A was reduced by 7 per cent (and by 3 per cent for DS) for a 5 per cent false-positive rate when using CRL rath er than LMP to date the pregnancy. This phenomenon is a consequence of an apparent reduction of gestational age when estimated by CRL in the aneuploid population. Further studies are required to evaluate whethe r CRL is an unbiased estimate of gestation for Down's syndrome pregnan cies.