Human chorionic gonadotropin (hCG) is a pregnancy hormone secreted by the 
placental synctiotrophoblast cell layer that has been linked to fetal growth and 
various placental, uterine and fetal functions. In order to investigate the 
effects of hCG on clinical endpoints, knowledge on reference range (RR) 
methodology and determinants of gestational hCG levels is crucial. Moreover, a 
better understanding of gestational hCG physiology can improve current screening 
programs and future clinical management. Serum total hCG levels were determined 
in 8195 women participating in the Generation R Study. Gestational age specific 
RRs using 'ultrasound derived gestational age' (US RRs) were calculated and 
compared with 'last menstrual period derived gestational age' (LMP RRs) and a 
model-based RR. We also investigated which pregnancy characteristics were 
associated with hCG levels. Compared to the US RRs, the LMP RRs were lower, most 
notably for the median and lower limit levels. No considerable differences were 
found between RRs calculated in the general population or in uncomplicated 
pregnancies only. Maternal smoking, BMI, parity, ethnicity, fetal gender, 
placental weight and hyperemesis gravidarum symptoms were associated with total 
hCG. We provide gestational RRs for total hCG and show that total hCG values and 
RR cut-offs during pregnancy vary depending on pregnancy dating methodology. 
This is likely due to the influence of hCG on embryonic growth, suggesting that 
ultrasound based pregnancy dating might be less reliable in women with high/low 
hCG levels. Furthermore, we identify different pregnancy characteristics that 
influence total hCG levels considerably and should therefore be accounted for in 
clinical studies.