The Influence of Fetal Sex on the Levels of Plasma Progesterone in the Human Fetus

Progesterone levels in the human fetoplacental unit at term were determined by a competitive protein binding method. Progesterone levels in the umbilical vein were always higher than in the umbilical artery. The concentration of progesterone in the umbilical vein of male and female fetuses was identical. However, the umbilical venous-arterial difference in progesterone levels was significantly greater in female than in male fetuses. The fetal genotype therefore may affect the fate of progesterone available to the fetus.