The early fetus gets the womb.

The human egg, once fertilized, apparently has only a short window of time in which to make it from a fallopian tube to the uterine wall. If the fertilized egg doesn't implant there within a week or so of ovulation, scientists find, the chances of a successful pregnancy begin to plummet.

Allen J. Wilcox of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., and his colleagues recruited 221 women who were about to stop using birth control because they wanted to become pregnant. From the concentrations of certain hormones in urine, the researchers could determine the day a woman ovulated. "We collected about 20,000 urine specimens. That's a lot of women collecting urine every morning and putting it into freezers," laughs Wilcox.

By also detecting the hormone chorionic gonadotropin in urine--the same method that home pregnancy tests use--the scientists could discern when an egg implanted. Cells that will become the placenta make this hormone to halt the menstrual cycle so that the woman doesn't shed the uterine lining and the implanted egg.

As the researchers describe in the June 10 NEW ENGLAND JOURNAL OF MEDICINE, they followed 189 women after conception. In almost all the women, the egg implanted 6 to 12 days after ovulation. The later the time of implantation, however, the more likely it became that the fetus would not survive its first 6 weeks. Indeed, no egg implanting after 12 days endured that initial period, let alone produced a live birth.

Animal studies indicate that the uterine wall becomes less receptive to implantation later in the menstrual cycle, which may explain the findings. Another possibility is that fertilized eggs that journey sluggishly to the uterus may have defects that make them less likely to survive. "It could well be both: There's a limited window of receptivity, and slower conceptuses are more likely to fail," says Wilcox.