A new study suggests that certain characteristics of a woman’s menstrual cycle may confer increased fertility, while others may lead to a greater risk of miscarriage.

To investigate whether the length of a woman’s cycle and her menstrual bleed are related to fertility and pregnancy outcome, Dr. Chanley M. Small of Emory University in Atlanta and colleagues followed 470 women for 1 year or the end of a pregnancy.

All of the women kept diaries on their menstrual cycles and collected urine samples for at least 2 days during each cycle. Thirty-eight percent of the women became pregnant during the course of the study, with a total of 207 pregnancies. Thirty percent of the pregnancies ended in miscarriage, according to a report in the journal Epidemiology.

Pregnancy was most likely to occur after cycles lasting 30 to 31 days, the investigators found, and 40 percent less likely after shorter cycles. Miscarriage was three times more common after cycles that were either shorter than or longer than 30 to 31 days.

Conception occurred most frequently after menstrual bleeds lasting 5 days, while spontaneous abortion was 60 percent less likely after periods lasting longer.

“Menstrual cycles may offer epidemiologists a noninvasive, immediate measure of reproductive health,” the researchers conclude, “and may be useful for studying a variety of host conditions, occupational and environmental exposures.”

SOURCE: Epidemiology, January 2006.