The aim of this study was to investigate the regulation of anti-Müllerian hormone (AMH) blood concentrations in mother and fetus during pregnancy. Serum concentrations of AMH, gonadotrophins, oestradiol and progesterone were measured in pregnant women in the first trimester and AMH concentrations in second-trimester fetuses, and these were compared in relation to the sex of the fetus. A total of 153 women undergoing elective termination of a first-trimester pregnancy and seven second-trimester pregnant women undergoing cordocentesis were included. Concentrations of AMH in the serum of first-trimester pregnant women were similar to non-pregnant women and were unrelated to the very high concentrations of human chorionic gonadotrophin and the undetectable concentrations of FSH and LH. Serum concentrations of oestradiol and progesterone were unrelated to the concentrations of AMH and the sex of the fetus. Serum concentrations of AMH of four, second trimester, male fetuses ranged from 64 to 92 ng/ml, whereas it was undetectable in female fetuses. It appears that AMH serum concentrations in first-trimester pregnant women seem to be independent of gonadotrophin concentrations and fetal sex. The concentration of AMH in the circulation of male fetuses is higher than previously reported and is a highly sensitive marker for fetal sex.