THE CONTROL OF OVULATION IN MOTHERS OF DIZYGOTIC TWINS

Dizygotic twinning is familial, suggesting that there may be an inheri ted abnormality of the control of ovulation that predisposes to double ovulation and, therefore, dizygotic twins. The present study examines 17 mothers of dizygotic twins (MODZT) and 8 control mothers of single tons by daily blood sampling throughout an entire menstrual cycle. Blo od samples were assayed for LH, FSH, estradiol, progesterone, and inhi bin. The process of follicular development was followed by transvagina l ultrasound. The pituitary LH response to iv GnRH was also assessed. Three of the 16 MODZT double ovulated during the study compared to non e of the 8 control mothers (P < 0.05). The number of small follicles ( < 6 mm) declined significantly in control women at midcycle, but not i n MODZT. There was no significant difference in serum FSH, LH, estradi ol, or inhibin levels between the 2 groups at any stage of the menstru al cycle. During the follicular phase, serum progesterone levels were significantly higher in MODZT. The response to GnRH stimulation was no t different between MODZT and controls. In conclusion, this study demo nstrates an increased tendency to double ovulate in MODZT that may be due to a reduced rate of atresia in advanced follicles. Furthermore, t he elevated progesterone levels in MODZT during the follicular phase s uggest altered intrafollicular steroidogenesis that is independent of gonadotropins.