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The gestation sac size in pregnancies resulting from in-vitro fertilization (IVF) and embryo transfer have been compared with those in spontaneous pregnancies. Small-for-dates gestational sac sizes were found in 36% of the IVF pregnancies. This proportion held for both singleton and multiple pregnancies. With increasing gestation beyond 8 weeks the gestation sac volume increasingly approached normal. In contrast to spontaneous conceptions, IVF pregnancies had a low rate of pregnancy loss once fetal heart movements were demonstrated, when the gestation sac size was small-for-dates. Small sac size in an IVF pregnancy may lead to the misdiagnosis of a failed pregnancy.

A retrospective analysis was carried out to assess the outcome of ovarian stimulation on in vitro fertilization when human chorionic gonadotropin (hCG) was administered after 5, 6, or 7 days of continuously rising plasma estradiol (E2). There was no significant difference in the number and size of large follicles in each group although the number of small follicles (less than 15 mm in diameter) decreased significantly after 7 days of E2 rise. After hCG injection in the 7-day group, the E2 level fell below the previous day's value in 40% of patients, whereas a similar fall was observed in only 16% of patients in the 5- and 6-day groups. In those cycles where a luteinizing hormone surge occurred, most surges were detected during the seventh day of E2 rise. The pregnancy rate was 31% when hCG was given after 6 days of rising E2, 21% after 5 days, and 14% after 7 days. In patients achieving pregnancy in the 6-day group, 53% of embryos were derived from leading follicles. In the 7-day group, only 15% of embryos associated with pregnancies were derived from leading follicles. These results strongly suggest that in stimulated cycles, hCG should be administered after 6 days of continuously rising E2. It is therefore postulated that 6 days of rising E2 represents a mean optimal period for follicular growth and oocyte maturation in stimulated cycles.