It is possible, though rare, for a woman to become pregnant naturally after age 45, according to the Baby Center Medical Advisory Board. A woman's egg supply decreases dramatically by age 40, and any remaining eggs are more likely to have issues that can result in miscarriage or birth defects.
Julia Johnson, chair of the reproductive endocrinology and infertility department at the University of Massachusetts, reports that fertility plummets for women in their 40s. Even a couple of years marks a huge drop in the likelihood of pregnancy after age 40. According to fertility expert James Goldfarb, using a donor egg raises the chance of pregnancy considerably.