May 10, 2006 The Menstrual Cycle. Mestruation (often referred to as the monthly period) is the cyclical shedding of tissue from the lining of the uterus. In the beginning of the menstrual cycle, hormones signal the endometrial lining to thicken in preparation for pregnancy. When a female becomes pregnant, the fertilized egg implants in the endometrial lining and grows. If no pregnancy occurs, the lining is shed (menstruation) and the cycle begins again. The average female starts menstruating around 12. However, girls may get their first period as young as 8 or as old as 16. The average menstrual cycle lasts 28 to 32 days, with 3 to 7 days of bleeding. Infertility The chance of a woman becoming pregnant in one month (without any pregnancy protection) is about 20 to 25 percent. Within three months, 57 percent of couples actively trying to have a baby achieve pregnancy. By 12 months, 87 percent of couples achieve pregnancy. Infertility is the inability to conceive after having unprotected intercourse for one year. According to the Hormone Foundation, about 3 million American couples are infertile. About 40 percent of cases are caused by problems in the female; 30 percent of cases involve male factors. In 20 percent of couples, infertility is caused by a combination of problems in the woman and man. In 10 percent of cases, the cause is unknown. Adding Up the Numbers: Effects of the Menstrual Cycle on Fertility Researchers at Emory University recently followed 470 women to look at the effects of menstrual cycle length and bleeding time on fertility. The participants kept daily diaries with information about their menstrual patterns and collected at least 2 urine samples during each cycle (to check for pregnancy). The women were followed for one year or until the end of a pregnancy. The investigators found women with menstrual cycles lasting 30 to 31 days had the highest rates of pregnancy. Women with shorter menstrual cycles were less likely to become pregnant. Women with shorter or longer cycles (i.e., lasting less than or longer than 30 to 31 days) who had gotten pregnant were more likely to experience miscarriage. The length of bleed time also appeared to be an important factor. Pregnancy rates were highest among women with 5-day bleeds. Women with bleed times greater than 5 days were less likely to have a miscarriage. Chanley Small, Ph.D., one of the study authors, says there are two possible reasons why menstrual cycle and bleed length may affect fertility. First, women with shorter or longer cycles may have underlying problems with egg release (i.e., something prevents the ovary from releasing the egg). Second, shorter menstrual cycles may be related to poorer overall egg quality. Small would like to see the study results replicated in other groups of women to confirm the effects of menstrual cycle characteristics on fertility. However, the results may point to some important signs for women, many of whom are delaying their childbearing and increasing their risk of having problems getting pregnant. Small recommends that women pay attention to their menstrual cycle characteristics. If menstrual cycles/bleed time become shorter or more variable, these could be signs of changes in fertility. AUDIENCE INQUIRY For general information on pregnancy and infertility: American College of Obstetricians and Gynecologists, public website, http://www.medem.com American Fertility Association, http://www.theafa.org American Pregnancy Association, http://www.americanpregnancy.org International Council on Infertility Information Dissemination (INCIID), http://www.inciid.org RESOLVE, http://www.resolve.org BIBLIOGRAPHY Crenin, M., et al., "How Regular is Regular?" Contraception, October 2004, Vol. 70, No. 4, pp. 289-292. Duijkers, I., et al., "Length of the Menstrual Cycle After Discontinuation of Oral Contraception," Gynecological Endocrinology, February 2005, Vol. 20, No. 2, pp. 74-79. "Frequently Asked Questions About Infertility," Bethesda: RESOLVE, downloaded from website (http://www.resolve.org), April 6, 2006. Frey, Keith, M.D., and Ketan Patel, M.D., "Initial Evaluation and Management of Infertility by the Primary Care Physician," Mayo Clinic Proceedings, November 2004, Vol. 79, No. 11, pp. 1439-1443. "The Menstrual Cycle," Bethesda: RESOLVE, downloaded from website (http://www.resolve.org), April 6, 2006. "Menstruation," Washington, DC: American College of Obstetricians and Gynecologists, downloaded from public website (http://www.medem.com), April 6, 2006. "Menstruation: A Journey Through Your Cycle," Irving: American Pregnancy Association, downloaded from website (http://www.americanpregnancy.org), April 6, 2006. Missner, Stacey, Sc.D., et al., "Reproductive History and Endometriosis Among Premenopausal Women," Obstetrics and Gynecology, November 2004, Vol. 104, No. 5, Pt. 1, pp. 965-974. Moffett, Ashley, et al., "Natural Killer Cells, Miscarriage, and Infertility," British Medical Journal, November 27, 2004, Vol. 329, No. 7477, pp. 1283-1285. Small, C., et al., "Menstrual Cycle Characteristic," Epidemiology, January 2006, Vol. 17, No. 1, pp. 52-60. "Understanding Your Most Fertile Time," New York: American Fertility Association, downloaded from website (http://www.theafa.org), April 6, 2006.