Influence of gravidity and foetal gender on the value of screening variables in the first trimester of pregnancy

Abstract

Objectives Combined screening for chromosome abnormalities in the first trimester of pregnancy is based on maternal age, nuchal translucency (NT) and biochemical markers (PAPP-A and free β-hCG). We sought to assess the value of the variables used in the combined screening strategy taking into account maternal gravidity and foetal gender. Study design Between July 1999 and December 2009, a total of 21,193 singleton pregnancies were screened for aneuploidy in the first trimester, in the Hospital Clínico San Carlos (Madrid, Spain). In the original database foetal gender data were available in 4370 euploid cases, and there were 2343 women with at least two consecutive pregnancies. We conducted a retrospective assessment of ultrasound and biochemical markers taking into account foetal gender and maternal gravidity, and evaluated the effect on the performance of screening, in terms of detection rates and false positive rates. Information on pregnancy outcome was obtained from the hospital's intranet medical records or by contacting the patient by telephone postpartum. Karyotype was ascertained by amniocentesis or chorionic villus sampling, and euploid status was assumed in newborns with normal phenotype. Student's t-tests (paired or unpaired as appropriate) were applied to the data, and the Bland–Altmann method was applied in evaluating individual differences in markers between successive gestations. Results PAPP-A decreased significantly between the first and the second pregnancy (p < 0.01). PAPP-A and free β-hCG values were significantly higher (p = 0.04 and p < 0.01 respectively) and NT was lower (p = 0.02) in pregnancies with a female foetus. Conclusions Correlations between the biochemical variables in relation to gravidity and foetal gender can introduce a bias in the calculated risk of chromosome abnormalities. Differences in NT measurements with respect to foetal gender do not seem to be of clinical importance. NT is independent of gravidity so routine use of NT compensates for the influence of these maternal–foetal variables on the values of biochemical parameters. Hence, the bias in overall combined screening is small.