Contralateral selection of dominant follicle favours pre-embryo development.

Ovulation was studied using vaginosonography in a total of 410 natural cycles of 123 women undergoing infertility treatment [267 intrauterine insemination (IUI) cycles of 103 women and 143 in-vitro fertilization (IVF) cycles of 50 women]. None of the women received ovarian stimulation. Each follicle was measured daily from 14 mm in diameter until formation of corpus luteum or oocyte retrieval. Contralateral ovulation as compared with the preceding cycle occurred in 57% of the 410 cycles. Contralateral ovulations occurred in 72% of cycles with a follicular phase < 13 days. In cycles with a follicular phase of > 14 days, ovulations occurred at random. The length of follicular phase in contralateral ovulation cycles (15.2 +/- 3.2 days) was significantly (P < 0.05) shorter than that of ipsilateral ovulation cycles (15.8 +/- 2.8). During the 57% contralateral ovulations in 143 IVF cycles, the rates of oocyte retrieval (89%), fertilization (69%), cleavage (90%) and embryo transfer (56%) were significantly higher than those of ipsilateral ovulations (69, 51, 64 and 23% respectively). The pregnancy rate of contralateral ovulations (9%) was also higher, though not significantly, than that of ipsilateral ovulations (3%), although the pregnancy rates per transfer were similar (16 and 14% respectively). The total pregnancy rate of both IUI and IVF was higher in contralateral than in ipsilateral ovulation cycles (8.1 and 4.0% respectively). The dominant follicles in contralateral ovulation cycles showed significantly higher oestradiol/androstenedione ratio (P < 0.025) and oestradiol/testosterone + androstenedione ratio (P < 0.025) and lower androstenedione (P < 0.05) than those of ipsilateral ovulation cycles. There was no significant difference in oestradiol, progesterone and testosterone. These results indicate that the dominant follicles in contralateral ovulation cycles are healthier than those of ipsilateral ones. Local intra-ovarian factors, e.g. from the corpus luteum, may negatively affect the health of the dominant follicle and the enclosed oocyte. Therefore contralateral selection of the dominant follicle in the succeeding cycle may favour pre-embryo development. The chance of conceiving during a natural cycle may be affected by the site of ovulation in the preceding cycle.