Researchers have been trying to figure out the characteristics of menstrual cycles that are most likely to result in pregnancy. A recent study of 470 women conducted by Emory University in Atlanta, Georgia, found that those with 30 to 31 day cycles and five days of menstrual bleeding were most likely to conceive. Shorter bleed times may reduce fertility because of a quick estrogen drop due to follicle deficiency or poor uterine lining buildup, the researchers suggest. But that’s certainly not to say that only women with cycles this long, who bleed for this many days, will get pregnant. Any woman with regular cycles of any length—indicating regular ovulation—should be able to conceive.