Adjustment of serum markers in first trimester screening.

First trimester combined screening can be performed using maternal serum pregnancy-associated plasma protein A, total human chorionic gonadotropin (hCG) and ultrasound measurement of nuchal translucency at 11-13 weeks of pregnancy. Our objective was to explore the effects of covariates on total hCG in the first trimester. First trimester total hCG levels were significantly increased in twins (median = 1.87 MoM), mildly increased in pregnancies achieved by in vitro fertilization (1.04 MoM) and decreased in smokers (0.80 MoM).