NEW YORK (Reuters Health) - As a woman's menstrual cycle shortens with advancing age, her odds of becoming pregnant decreases, Swedish investigators report in the journal Fertility and Sterility.

Dr. Thomas Brodin of Malarsjukhuset in Eskilstuna, Sweden, and colleagues analyzed successful pregnancy and delivery rates in 6,271 in vitro fertilization cycles in terms of menstrual cycle length. Their objective was to determine if cycle length could be used as a marker for fertility potential.

"Increasing age was associated with a subtle shortening of (average) menstrual cycle length. The (average) shortening of menstrual cycle length is about two days from the twenties to the forties," Brodin said in an interview with Reuters Health.

After accounting for age, the menstrual cycle length was directly related to the odds of pregnancy and delivery, the team found. "The chance of delivery after (in vitro fertilization) was almost doubled for women with a menstrual cycle length longer than 34 days compared with women with a menstrual cycle length shorter than 26 days."