Serum free β-human chorionic gonadotropin in the three trimesters of pregnancy: effects of maternal characteristics and medical history.

OBJECTIVE To define the contribution of maternal variables which influence the measured level of maternal serum free β-human chorionic gonadotropin (β-hCG) in screening for pregnancy complications. METHODS Maternal characteristics and medical history were recorded and serum free β-hCG was measured in women with a singleton pregnancy attending for three routine hospital visits at 11 + 0 to 13 + 6, 19 + 0 to 24 + 6 and 30 + 0 to 34 + 6 weeks' gestation. For pregnancies delivering phenotypically normal live births or stillbirths ≥ 24 weeks' gestation, variables from maternal demographic characteristics and medical history that are important in the prediction of free β-hCG were determined from a linear mixed-effects multiple regression. RESULTS Serum free β-hCG was measured in 94 985 cases in the first trimester, 7879 in the second trimester and 8424 in the third trimester. Significant independent contributions to serum free β-hCG were provided by gestational age, maternal weight, age and racial origin, cigarette smoking, method of conception, diabetes mellitus and family history of pre-eclampsia (PE) in the mother of the patient. The effects of some variables were similar and those for others differed in each trimester. Random-effects multiple regression analysis was used to define the contribution of maternal variables that influence the measured level of serum free β-hCG and express the values as multiples of the median (MoMs). The model was shown to provide an adequate fit of MoM values for all covariates both in pregnancies that developed PE and in those without this pregnancy complication. CONCLUSIONS A model was fitted to express measured serum free β-hCG across the three trimesters of pregnancy as MoMs after adjusting for variables from maternal characteristics and medical history that affect this measurement.