Fertility Findings Are Confirmed : Pregnancy: Research shows that the window of opportunity is shorter than previously thought, making ovulation kits more valuable.

The finding that a woman’s fertility window is much shorter than believed--reported Thursday in the New England Journal of Medicine--is expected to give a boost to the use of home ovulation predictor kits, although the kits predict for a much shorter period.

Nearly all pregnancies among 221 women studied occurred during a six-day period ending on the day of ovulation, proving the optimal days for conceiving are far fewer than commonly believed.

Home ovulation kits can predict ovulation from about 12 to 40 hours before the event.

“Using the home kits to time intercourse is an excellent way of increasing the chance of conception,” says Dr. T. Murphy Goodwin, director of maternal-fetal medicine at Good Samaritan Hospital. “If anything [the study finding] will make the kits more valuable.”

Even though the fertility window is about six days, the chances of conceiving six days before are lower than during a time closer to ovulation, Goodwin says.

“The kit [results] would put you smack in the most fertile part of the window. If you want to have a 33% chance of conceiving on a given cycle, you need to be having intercourse on the day of ovulation [according to the study].”

Home ovulation kits are sold over-the-counter under names such as OvuQUICK, OvuKIT, Conceive and Answer.

Dr. Irwin Frankel, a Los Angeles obstetrician on staff at Cedars-Sinai Medical Center and Century City Hospital, says the study casts into stone what physicians have long suspected about the shorter fertility window. “They’re not saying anything new,” he says.

The study findings also suggest that other methods of predicting ovulation will be less valuable for women hoping to get pregnant, Goodwin adds.

One such method is mittelschmerz--the pain of ovulation, which some women trying to get pregnant use as a signal to have intercourse.

“If she actually has ovulated at that time, the chances of fertility drop off rapidly [right] after that,” Goodwin says.