Mid-pregnancy, perinatal, and neonatal reproductive endocrinology: a prospective cohort study in twins and singleton control subjects.

<bold>Objective: </bold>To answer the questions: Are perinatal reproductive hormone profiles different in case of a twin compared with a singleton pregnancy? Are reproductive endocrine profiles of twin girls influenced by their male co-twin and vice versa?<bold>Design: </bold>Prospective cohort study from January 2004 to October 2009.<bold>Setting: </bold>Not applicable.<bold>Patient(s): </bold>A total of 204 mothers of twins and 248 singleton control subjects, aged >18 years, pregnant with a twin or singleton and no endocrine disease or malignancy.<bold>Intervention(s): </bold>Blood samples were collected at mid-gestation from the mother and at delivery from the mothers and the umbilical cords. Estrogens, androgens, sex hormone-binding globulin, progesterone, and gonadotropins were measured.<bold>Main Outcome Measure(s): </bold>Hormonal profiles were compared between singletons and twins, different types of twins, and opposite-sex and same-sex twins.<bold>Result(s): </bold>Estrogen and progesterone concentrations were higher in mothers of twins compared with singletons, but twin babies had lower estrogen and progesterone concentrations at birth. Opposite-sex twin girls did not have higher androgens in cord blood compared with same-sex twin girls. Boys of an opposite-sex twin had lower luteinizing hormone concentrations compared with dizygotic twin boys with a brother as a co-twin.<bold>Conclusion(s): </bold>Children from a twin are not overexposed to sex steroids at the time of birth, despite higher concentrations in their mothers, and girls from opposite sex twins do not show androgenic influences from their male co-twin. The female co-twin may influence the hypothalamic-pituitary-testicular axis of her brother via central inhibition. [ABSTRACT FROM AUTHOR]