Time interval from human chorionic gonadotrophin (HCG) injection to follicular rupture

The aim of this study was to estimate the time interval from human chorionic gonadotrophin (HCG) injection to follicular rupture. Furthermore, it was observed whether there was any effect on the pregnancy rate if the insemination was performed at the time of follicular rupture. In a programme of intrauterine insemination 37 consecutive cycles in 32 patients were monitored after stimulation with clomiphene citrate. HCG was administered by i.m. injection when a leading follicle of at least 18 mm in diameter was observed sonographically. All patients were monitored by sonography with 1 h intervals from 32 h after HCG injection until the first rupture of a follicle. Insemination was performed immediately after the first follicular rupture. The pregnancy rate was 16% (5/32). In 66%, the largest follicle was the first to rupture. The mean time interval from HCG administration to first follicular rupture was 38.3 h (SEM = 0.54; range = 34-46). Our findings support the concept that ovulation occurs about 38 h after HCG administration. The pregnancy rate was within the normal range, although insemination was performed at the time of follicular rupture. The largest follicle was not always the first to rupture.