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An ovary's ovulation pattern influences the pregnancy rate and the gender of the offspring from its eggs, according to research from assisted reproduction clinicians in Japan.
Dr Misao Fukuda from M&K Health Institute in Hyogo and colleagues had previously shown that anovulation during two menstrual cycles enhances the pregnancy potential of oocytes obtained during the subsequent third cycle.
In a new study reported online in Fertility and Sterility, they evaluated whether the ovulation pattern of three consecutive menstrual cycles in which ovulation was unilateral affected the chance of achieving pregnancy and the sex of the offspring.
The study
They recruited 335 women undergoing in vitro fertilisation (IVF) or intrauterine insemination (IUI).
The oocyte retrieval rate, fertilisation rate, and rate of pre-embryo formation were significantly higher after contralateral ovulation patterns (left-right or right-left for the last two cycles) than after ipsilateral ovulation patterns for the last two cycles.
When all three cycles were included, the pattern of left-left-right ovulation had the highest pregnancy rate (27%) and live birth rate (22%) when compared with all the other possible ovulation patterns.
Moreover, the left-left-right ovulation pattern yielded a significantly higher ratio of male-to-female offspring (0.762) than the patterns.
Speculation
Dr Fukuda speculated as to why this might occur. "The oocyte following an LLR ovulation pattern, freed from the suboptimal effects from the dominant follicle and/or the corpus luteum during two previous menstrual cycles, is well matured and prone to be receptive to Y-sperm."
Y-sperm, which are smaller than X-sperm, "could easily penetrate the zona pellucida of the oocyte following LLR," Dr Fukuda said.
According to previous estimates, it takes three menstrual cycles for human follicular development to progress from the preantral stage to the preovulatory follicle and ovulation; those estimates at least partially support Dr Fukuda's notion.
"Large scale observational studies are necessary to confirm that the present finding is correct," Dr Fukuda cautioned. "Next, comparison of health status of preantral follicles and/or early stage of antral follicles from between the ovaries with and without dominant follicle or corpus luteum would be of interest."
Dr Fukuda added, "It appears that when human reproduction is under pressure or unfavorable conditions more females tend to be born. Y-sperm or XY embryo might be more fragile, vulnerable, and sensitive than X-sperm or XX embryo."
(Reuters Health, Will Boggs, April 2011)
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