Serum concentrations of human chorionic gonadotrophin (HCG), Schwangerschaftsprotein 1 (SP-1), pregnancy-associated plasma protein A (PAPP-A), progesterone and oestradiol were measured at weekly intervals between the fifth (embryo transfer plus 3 weeks) and 13th week of gestation during the first trimester of pregnancies achieved following in-vitro fertilization (IVF) and embryo transfer in a group of women who delivered before (n = 8) or at term (n = 52). Those women who had a preterm delivery had significantly lower concentrations of PAPP-A (weeks 7-13; P = 0.0001-0.028) and SP-1 (weeks 6-8 and 10-12; P = 0.004-0.04). After correction of birth weight for sex and gestational age at delivery, preterm delivery was found not to be associated with growth retardation. However, comparison of the circulating concentrations of the substances analysed in mothers who delivered babies of < 85% of the 50th centile of the normal range of birth weight for a given gestational age and sex, with those who delivered babies of > 85% revealed that the concentrations of HCG (P = 0.012-0.04 on weeks 6-9) and SP-1 (P = 0.003-0.03 on weeks 7, 9-13) were significantly lower in the former group. Weak, inconsistent associations were found between the circulating concentrations of HCG, SP-1 and PAPP-A and both corrected birth weight and gestational age at delivery. Thus, both the gestational age at delivery and low birth weight may be related to impaired placental development/function during the first trimester.