If you're on the Pill or another form of hormone-altering birth control, like the Patch or the Ring, it may be masking symptoms of infertility. These methods bring on a "false" period, which is bleeding that's not a response to ovulation. "So if you develop a condition that tends to cause irregular or missed periods, you may not realize it," says Veronica Ravnikar, M.D., chair of obstetrics and gynecology at the South Shore Hospital in South Weymouth, MA.

These include some thyroid disorders, common in women, especially after having a baby; premature ovarian failure, the rare onset of menopause before age 40; and polycystic ovary syndrome (PCOS), the most common cause of infertility among American women. PCOS affects as many as 10 percent of women of childbearing age and is marked by excess production of male hormones and insulin problems (which increase diabetes risk, especially if you do become pregnant) and symptoms including acne, weight gain (especially around the waist), unwanted body and facial hair, and pelvic pain.

If you experience any of these while on hormonal birth control  -- or had them before you started on it  -- let your doctor know. He may want to test you for PCOS by measuring your male-hormone levels even before you try to conceive your next child, so that you can start treatment (often with Metformin, a drug that improves insulin sensitivity), sooner rather than later.