Serum albumin levels decrease during pregnancy while the concentration of most other maternal serum proteins of hepatic origin remain stable or increase. In a study of 289 women, most maternal characteristics such as race, age, smoking, a history of previous low birth-weight, infant sex and gestational age at delivery were not related to maternal serum albumin levels at 18 or 30 weeks' gestational age. The degree of maternal obesity significantly correlated with the concentration of albumin. There was a significant negative correlation in individual women between maternal serum levels of albumin and a-fetoprotein, with high levels of maternal serum a-fetoprotein predicting lower levels of albumin. We hypothesize that there may be a negative feedback effect of α-fetoprotein of fetal origin on the maternal production of albumin during pregnancy. © 1993.