Postponing the hCG administration until after the IUI instead of injecting it 24-32 hours before IUI resulted in a significantly increased pregnancy rate. The other independent factors affecting the IUI cycle outcome in this study were the number of follicles > 16 mm and the sperm count.
In most studies that have evaluated the outcome in IUI cycles, the insemination has been performed 24-36 hours following hCG administration [1, 4]. This practice is based on data indicating that, in natural cycles, the ovulation takes place 32 hours (range 24-56 hours) after the onset of the luteinizing hormone (LH) surge , whereas in stimulated cycles, it takes place approximately 36-38 hours after the hCG injection . As the current assumption is that the oocytes are fertilisable for only 12-16 hours  and the spermatozoa survive only for a limited period of time in the female reproductive tract [8–10], it is rational to schedule the insemination to the time of expected ovulation, i.e., 24-36 hours after the administration of hCG.
In 1995, Wilcox et al. published a study of 221 healthy women who were planning to become pregnant . After stopping birth-control methods, the women collected daily urine specimens and kept daily records of their sexual intercourses. Oestrogen and progesterone metabolites were measured from the urine samples to estimate the day of ovulation. The authors observed that conception occurred only when intercourse took place during a six-day period that ended on the estimated day of ovulation . This finding suggests that the chances to conceive in the natural cycle diminish considerably after ovulation and that, preferably, the spermatozoa should be available in the reproductive tract before ovulation occurs. It also suggests that spermatozoa may survive for several days after intercourse, as women whose last intercourse took place 5-6 days before ovulation did conceive.
The results of the study by Wilcox et al. (1995) do not totally support the current practice in IUI that hCG should be administered before the insemination but rather that it should be injected after the insemination. In our study, we observed an increase of 80% in pregnancy rate by postponing the hCG injection after the IUI. The change in hCG administration ensured that the spermatozoa were already present in the reproductive duct before ovulation took place, similar to the situation in successful natural cycles . Since sexual intercourse was allowed a couple of days before the insemination and also after it, spontaneous conception is possible although unlikely, since all of the couples had at least one year infertility even before the start of basic investigations.
Our results also confirmed earlier findings that the number of follicles > 16 mm  and sperm count  are independent factors affecting the IUI cycle outcome.