Researchers report in today's New England Journal of Medicine that the highest probability of pregnancy each month occurs during a six-day period ending on the day of ovulation -- a time frame several days earlier and considerably shorter than conventional wisdom dictates.

"The kind of thing that obstetricians tend to tell their patients is that their most fertile period is a few days before ovulation and a few days afterward," said Allen J. Wilcox, who headed the research team at the National Institute of Environmental Health Sciences (NIEHS) in Research Triangle Park, N.C. "But our data seem to indicate that the day of ovulation is the end of the fertile time, not the middle of it."

"I think this is a very important and helpful article for infertile couples," said Howard A. Zacur, director of the reproductive endocrinology division at Johns Hopkins Medical Institutions. "It can alleviate a bit of the pressure {they} sometimes feel when they think they must have intercourse on a certain day or all is lost." Equally reassuring, he said, is the finding that even though sperm may have been present in the reproductive tract for a day or two before ovulation, "that does not mean that one risks a pregnancy loss" from possible aging or degradation of the sperm.