REGULATION OF CORPUS-LUTEUM FUNCTION

The effects have been studied of different ovulation induction regimens [either clomiphene citrate or buserelin in combination with human menopausal gonadotrophin (HMG)] on the circulating concentrations of progesterone, oestradiol, relaxin and human chorionic gonadotrophin (HCG) during the first trimester of pregnancy. Ovulation induction with clomiphene resulted in elevated concentrations of gonadotrophins in both phases of the cycle, while during ovulation induction with buserelin, gonadotrophin concentrations were elevated in the follicular phase only. The concentrations of all corpus luteum products were greater in clomiphene pregnancies compared with spontaneous pregnancies, but only oestradiol and relaxin concentrations were greater in clomiphene pregnancies compared with buserelin pregnancies. The concentrations of HCG were significantly reduced in clomiphene pregnancies compared to natural pregnancies. Relaxin concentrations were significantly higher from 7 weeks gestation in buserelin compared with spontaneous pregnancies, while progesterone, oestradiol and HCG concentrations were not consistently different. Consistent associations were found between relaxin and HCG concentrations in spontaneous pregnancies and between the concentrations of relaxin and both progesterone and oestradiol in pregnancies achieved after ovulation induction. These data suggest that (i) given the similarity in the circulating concentrations of HCG, the relatively lower circulating gonadotrophin concentrations during the luteal phase of the cycle of conception result in reduced circulating concentrations of oestradiol and relaxin; while in the case of relaxin this effect is partially reversible, there is no evidence that this is so for oestradiol; (ii) synthesis of progesterone in the corpus luteum is less affected by lower concentrations of gonadotrophins during the luteal phase; (iii) ovulation induction with clomiphene results in pregnancies with lower concentrations of HCG, suggesting that trophoblast function may be impaired; and (iv) corpus luteum function is linked with placental steroidogenesis.