What you've heard about age and fertility is true--as a woman ages, her ability to conceive declines. This gradual decline in fertility starts very subtly in the late 20s to age 35 but then becomes more pronounced as age approaches 40 and older.

In one French study, researchers found that the pregnancy rate over one year for women younger than 31 was 74%. For women between the ages of 31 and 35 the rate declined to 62% and to 54% for women beyond 35 years of age. In another study, 87% of women age 45 and older were no longer able to bear children. But the ticking of the biological clock is not as inexorable as it once seemed. Medicine and technology are developing ways for women to improve their chances of conceiving. This article will try to explain not only why fertility declines with advancing age, but also how assisted reproductive technology can improve a woman's chances of becoming pregnant.

The reasons for the decreased fertility rate with age is multi-fold. Many women, married or unmarried, are waiting longer before attempting pregnancy. As sexually active women grow older, the likelihood that they might be exposed to sexually transmitted infections increases. These infections (such as chlamydia or gonorrhea) can permanently scar the pelvic organs which can hinder a woman's ability to become pregnant. The chance that a woman might experience fertility related complications from endometriosis or adenomyosis (disorders which involve uterine lining cells--endometrial cells--implanting in abnormal locations in the pelvis) increases with age as well.

One of the most important explanations for age-related infertility in women is the declining number of genetically normal available eggs. The peak number of eggs (also known as oocytes) is achieved long before women even consider becoming pregnant: when a female fetus is 4-5 months old, still in the mother's uterus, it possesses up to 6-7 million eggs. By birth, this number drops to 1-2 million and declines even further when, at the start of puberty in normal girls, there are 300,000-500,000 eggs. Several hundred oocytes are lost during the 3-4 decades a woman has regular menstrual cycles through the monthly development and ovulation of an oocyte. Many other oocytes are lost through triggered, natural cell death. When a woman reaches her mid- to late 30s, when she has about 25,000 eggs left in her ovaries, the loss rate of oocytes accelerates. In addition, as a woman ages the ability of her oocytes to divide and distribute the genetic contents normally declines. The likelihood that an oocyte with an abnormal number of chromosomes will be fertilized increases with age.