Hypothetical risks of twinning in the natural menstrual cycle.

Dizygotic (DZ) twinning is likely the result of multiple ovulation upon multiple ongoing follicle growth. Well known conditions with an increased incidence of DZ twinning are a hereditary trait, high maternal age and ovulation induction treatments. These conditions are clearly related to elevation of Follicle Stimulating Hormone (FSH) at the time of peri-menstrual follicle recruitment. In this contribution, we summarize certain other circumstances that may occur in normal women that could increase twinning rate based on the assumption that the elevation of FSH levels is the principal underlying mechanism in DZ twinning. It is hypothesized that recovery from prolonged hypothalamic amenorrhea in part mimics hormonal sequelae of puberty characterized by temporary overshoot of FSH levels. By deduction, such conditions are recovery from lactational amenorrhea, recovery from use of oral contraceptives and recovery of weight loss amenorrhea. According to the literature, these conditions indeed carry the risk of DZ twinning and all show temporary elevated FSH levels. However, prospective experiments will be needed to prove the relation between this type of DZ twinning and possible patterns of serum FSH that mimic puberty under these circumstances.