As a result of recent cultural shifts that have led women to delay pregnancy in order to pursue education and careers, many women in their thirties who are only beginning to think about starting families are left wondering "When is a woman most fertile?" This is a question that really has two answers, one regarding when during her life cycle a woman is most fertile and the other regarding when in her menstrual cycle a woman is most fertile.

Age and Fertility

Female fertility peaks around age 27 and begins to decline after 30. By 35, the quality of eggs produced by her ovaries begins to decline significantly, and by 40, women have less than a 10 percent chance of carrying a pregnancy conceived with their own eggs to term. Pregnancies conceived with older eggs also have a much higher chance, up to 1000 percent higher, of having chromosomal abnormalities, which can lead to conditions like Down's syndrome.

Ovulation and Fertility

If you are trying to get pregnant, one of the easiest ways to boost your chances is to track your cycle to determine when you're ovulating. Most women begin ovulation between about 11 to 14 days after the start of menstruation, but because cycles can vary, most doctors recommend that you plan sexual intercourse regularly between days seven and 20 of the menstrual cycle. Because sperm lives for up to five days in the female body, and the egg lives for up to 24 hours after it's released from the ovary, it's not necessary to have sex every day during this period, and studies show that having sex every other day, or even every third day, can be as effective.

Signs to Look For

If you have an irregular period, it may be necessary for you to look for specific signs of ovulation or find a more accurate way to track your cycle in order to ensure intercourse during the peak reproductive period. One of the easiest ways to gauge impending ovulation is to look for changes in your cervical fluid, which will become slippery and stretchy right before your most fertile periods. You can also use a basal thermometer to track changes in your body temperature that will indicate when you are about to ovulate. When you begin to ovulate, you temperature rises and then stays elevated until the end of the ovulation cycle.