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Planet Earth

Conceptual Shift

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The conventional wisdom on making babies is that the best time to try is between three days before the woman ovulates and about two days afterward. This standard advice--based on a decades-old survey of British couples--seems to be flawed, according to a new study by the National Institute of Environmental Health Sciences. A woman’s monthly fertile period is indeed around six days long, the study concludes, but it ends on the day of ovulation, when the egg is released from the ovary. Intercourse after that is very unlikely to lead to conception.

Why has it taken so long for scientists to discover this elementary fact of human biology? The way medical research works is that we’re pretty good at studying diseased people, but what happens in ordinary healthy people is much more elusive, says epidemiologist Allen Wilcox, one of the authors of the current study. One flaw in earlier studies was that they tried to gauge the time of ovulation from a slight rise in body temperature--a notoriously imprecise measure, says Wilcox.

In their study of 221 women who were planning to become pregnant, Wilcox and his colleagues had each one collect a urine sample every morning, beginning on the day she ceased using birth control and continuing until the eighth week of pregnancy, or for up to six months if no pregnancy resulted. At the same time, the women recorded whether they had had sexual intercourse in the previous 24 hours.

The researchers determined when ovulation had occurred by analyzing the urine for evidence of the hormonal changes--an abrupt drop in estrogen and a rise in progesterone--that are triggered when the ovary releases an egg. The presence in the urine of human chorionic gonadotropin, a hormone released by the embryo, revealed when a woman had become pregnant.

Of the 192 pregnancies that began during the study, all resulted from intercourse that occurred within the six days that ended with the day of ovulation, with the likelihood of conception increasing as the days passed. Intercourse that took place later than this produced no pregnancies. Although sperm can apparently survive in the uterus or the oviducts for at least five days, Wilcox suspects that the egg may break down within a day of ovulation. Alternatively, he speculates, the mucus secreted by the cervix, through which sperm must migrate to get to the egg, may become impermeable.

One happy implication of the study, says Wilcox, is that a woman planning to become pregnant should not wait to have sex until the kit she bought in a drugstore tells her she has ovulated--because then she may miss the window of opportunity altogether. And though previous research has shown that frequent ejaculation can decrease the sperm count, Wilcox’s study showed that daily sex produced no loss of fertility. For couples who are having intercourse on the day of ovulation, their chances of conceiving on that day don’t seem to be any worse if they also had intercourse on every day prior to that going back for a week, says Wilcox. There seems to be no disadvantage to frequent intercourse.

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