Serum levels of immunoreactive inhibin, FSH, and LH in human infants at preterm and term birth.

Serum levels of immunoreactive inhibin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) were determined in 112 fetal cord blood samples obtained at birth between 26 and 40 weeks of gestation. High levels of inhibin immunoreactivity were detected in all samples. Between the gestational age of 26 and 28 weeks, the levels (mean +/- SE) were higher (p less than 0.05) in male (21.6 +/- 1.0 U/ml; n = 12) than in female (12.8 +/- 0.2 U/ml; n = 12) fetuses. With ongoing gestation, the serum inhibin immunoreactivity decreased and was found to be similar in male (12.1 +/- 0.3 U/ml; n = 13) and female (9.1 +/- 0.7 U/ml; n = 8) fetuses at term. Serum FSH and LH levels were elevated at the beginning of the 3rd trimester of pregnancy and decreased with ongoing gestation to undetectable values at term birth. Between 26 and 32 weeks of gestation, the FSH levels were higher in females (p less than 0.02), whereas the LH levels were higher in males (p less than 0.01). These observations suggest that in the human fetus the pituitary-gonadal axis is active and presents sexual dimorphism; both characteristics are pronounced early during the 3rd trimester of gestation and decrease towards term.