Ovary and Ovulation: Contralateral selection of dominant follicle favours pre-embryo development

Ovulation was studied using vaginosonography in a total of 410natural cycles of 123 women undergoing infertility treatment267 intrauterine insemination (IUI) cycles of 103 women and143 in-vitro fertilization (IVF) cycles of 50 women]. None ofthe women received ovarian stimulation. Each follicle was measureddaily from 14 mm in diameter until formation of corpus luteumor oocyte retrieval. Contralateral ovulation as compared withthe preceding cycle occurred in 57% of the 410 cycles. Contralateralovulations 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 phasein contralateral ovulation cycles (15.2 ± 3.2 days) wassignificantly (P < 0.05) shorter than that of ipsilateralovulation cycles (15.8 ± 2.8). During the 57% contralateralovulations 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 contralateralovulations (9%) was also higher, though not significantly, thanthat of ipsilateral ovulations (3%), although the pregnancyrates per transfer were similar (16 and 14% respectively). Thetotal pregnancy rate of both IUI and IVF was higher in contralateralthan in ipsilateral ovulation cycles (8.1 and 4.0% respectively).The dominant follicles in contralateral ovulation cycles showedsignificantly higher oestradiol/androstenedlone ratio (P <0.025) and oestradlol/testosterone + androstenedione ratio (P< 0.025), and lower androstenedione (P < 0.05) than thoseof ipsilateral ovulation cycles. There was no significant differencein oestradiol, progesterone and testosterone. These resultsindicate that the dominant follicles in contralateral ovulationcycles are healthier than those of ipsilateral ones. Local intra-ovarianfactors, e.g. from the corpus luteum, may negatively affectthe health of the dominant follicle and the enclosed oocyte.Therefore contralateral selection of the dominant follicle inthe succeeding cycle may favour pre-embryo development. Thechance of conceiving during a natural cycle may be affectedby the site of ovulation in the preceding cycle.