by Mary Kittel with Deborah Metzger, M.D., Ph.D.

Up until 2001, there were no clinical guidelines to characterize our reproductive life span. Finally, representatives from esteemed organizations, including the American Society for Reproductive Medicine (ASRM), the National Institutes of Health, and the North American Menopause Society, pooled their knowledge and developed the STRAW system. STRAW stands for the Stages of Reproductive Aging Workshop in which it was developed, under the direction of Dr. Soules.

The beauty of STRAW is that you can get a sense of what stage you are in based on your menstrual patterns, symptoms, and the overall characteristics of a stage, rather than just a certain hormone range or chronological age. Looking at the whole picture makes it clearer that reproductive capacity is unique for each individual, explains Dr. Soules. After all, there are a great number of variables in each woman's genetic makeup, health, and lifestyle habits that carry her more quickly or slowly from one reproductive stage to another.

Even the stages designated by STRAW aren't set in stone. "While most women will progress from one stage to the next, some will seesaw between stages or skip a stage altogether," says Dr. Soules. It's also important to keep in mind that although normal menopause occurs between 42 and 58, the age ranges in STRAW are based on the average age of 50 (so you can feasibly be up to 8 years ahead of or behind the curve).

Note that while the stages are based on the STRAW model, our experts not affiliated with STRAW have also commented on fertility issues to be aware of at each stage.

Early Reproductive Stage

Approximate age: Puberty to age 20

Hormone levels: Your FSH and estrogen are within normal range (although you probably have no reason to have them tested).

Characteristics: Following your first period, your menstrual cycles may take several years to fall into the 28- to 31-day pattern that you can expect for the majority of your reproductive years. Your hormone feedback system may not be strong enough to always induce ovulation, which is one reason why sexually active women in this stage are slightly less fertile than their more mature sisters.

Fertility issues: "Women in this generation are the only group whose smoking rates are on the rise. That's very bad news, because you may be killing off without any trace a substantial number of eggs that you'll wish you had 15 years down the line, especially if you expect to delay childbearing," says Dr. Tilly.

Stage-specific advice: Single, teen mothers are the most economically disadvantaged group in the nation. "Planning childbearing with your career in mind and waiting for a supportive partner are perfectly legitimate reasons to delay childbearing," says American culture scholar Margaret Marsh, Ph.D., professor of history and dean of the faculty of arts and sciences at Rutgers University in Camden, New Jersey, and coauthor of The Empty Cradle: Infertility in America from Colonial Times to the Present.

Middle Reproductive Stage

Approximate age: 20 to 38

Hormone levels: Your FSH and estrogen are within normal range (you might want to get a baseline test at around age 35).

Characteristics: Your menstrual cycles should be regular throughout this stage. One percent of the female population, however, experiences menopause before the age of 40, where FSH levels prematurely increase and menstrual cycles stop.

Fertility issues: This is the longest of all the stages. It's actually broken into three segments. The first 7 years are your peak fertility time; the next third marks your first slight fertility decline, followed by the beginning of your second fertility decline, around age 35. (The sharpest statistical drop in fertility, however, occurs at the age of 45, according to the U.S. Census Bureau.)

Stage-specific advice: Sexually transmitted diseases (STDS) are highest among women in their twenties, and they're the leading cause of infertility for this generation. "The greatest long-term fertility advice we doctors can give is to avoid STDs by practicing safe sex," says Wanda Ronner, M.D., a gynecologist at Pennsylvania Hospital of the University of Pennsylvania Health System in Philadelphia and coauthor with Dr. Marsh of The Empty Cradle: Infertility in America from Colonial Times to the Present.

Late Reproductive Stage

Approximate age: 38 to 42

Hormone levels: Your FSH levels may have jumped from around 5 mIU/mL to closer to 15 or 20. If a test indicates that your FSH levels are normal and you're already 40, have the test taken a second time, since levels in this stage tend to fluctuate from month to month.

Characteristics: Although your FSH levels may have increased, you probably still ovulate and menstruate on your usual 28- to 31-day schedule. But since your follicles are getting less responsive to FSH, they may produce less estrogen. As the body's estrogen receptors take note, you may become more sensitive to extreme temperatures or even experience hot flashes. Since lower estrogen levels affect the elasticity and acidity of the vagina, you may need a lubricant during sex or develop more yeast and bladder infections than you did in younger years. Even if you didn't get PMS in the past, you're now more likely to experience mood swings and discomfort around the time of your period.