A 45-year-old woman is less likely to conceive than a 40 year old, who is less likely to conceive than a 35 year old. However, the time when a woman can conceive varies widely from woman to woman. As a woman ages not only does the likelihood of conceiving decline, the likelihood of having a miscarriage increases. At age 30, the rate of early miscarriage is 15 percent, at 40 - 44 it is 51 percent, and greater than age 45 it is 93 percent.
In our offices, we gynecologists tend to tell our patients that as long as they are having menstrual periods they can conceive. We recommend that all women who are sexually active and menstruating should use contraception. Of course a 51-year-old woman is quite unlikely to conceive, but there is a possibility of a pregnancy, most likely not viable, and thus it is most prudent to use contraception.
There are many safe contraceptive options for women in their 40s and 50s, including an IUD or a very low-dose birth control pill. The options are best discussed with your doctor who can take your own health history into consideration.