Ovulation was studied using vaginosonography in a total of 410 natural
cycles of 123 women undergoing infertility treatment [267 intrauterin
e insemination (IUI) cycles of 103 women and 143 in-vitro fertilizatio
n (IVF) cycles of 50 women], None of the women received ovarian stimul
ation. Each follicle was measured daily from 14 mm in diameter until f
ormation of corpus luteum or oocyte retrieval. Contralateral ovulation
as compared with the preceding cycle occurred in 57% of the 410 cycle
s, Contralateral ovulations occurred in 72% of cycles with a follicula
r phase <13 days, In cycles with a follicular phase of >14 days, ovula
tions occurred at random. The length of follicular phase in contralate
ral ovulation cycles (15.2 +/- 3.2 days) was significantly (P < 0.05)
shorter than that of ipsilateral ovulation cycles (15.8 +/- 2.8), Duri
ng the 57% contralateral ovulations in 143 IVF cycles, the rates of oo
cyte retrieval (89%), fertilization (69%), cleavage (90%) and embryo t
ransfer (56%) were significantly higher than those of ipsilateral ovul
ations (69, 51, 64 and 23% respectively). The pregnancy rate of contra
lateral ovulations (9%) was also higher, though not significantly, tha
n that of ipsilateral ovulations (3%), although the pregnancy rates pe
r transfer were similar (16 and 14% respectively), The total pregnancy
fate of both IUI and IVF was higher in contralateral than in ipsilate
ral ovulation cycles (8.1 and 4.0% respectively), The dominant follicl
es in contralateral ovulation cycles showed significantly higher oestr
adiol/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 result
s indicate that the dominant follicles in contralateral ovulation cycl
es are healthier than those of ipsilateral ones, Local intra-ovarian f
actors, e.g. from the corpus luteum, may negatively affect the health
of the dominant follicle and the enclosed oocyte, Therefore contralate
ral selection of the dominant follicle in the succeeding cycle may fav
our pre-embryo development, The chance of conceiving during a natural
cycle may he affected by the site of ovulation in the preceding cycle.