Women typically have a decrease in fertility beginning in their early 30s.  On average, a woman in her mid-30s or older will take longer to conceive than a younger woman.  At age 30 to 35, fertility is 15 to 20 percent below maximum.  From age 35 to 39, the decrease is 25 to 50 percent.  From 40 to 45, the decrease is 50 to 95 percent.

Age-related declines in fertility may be at least partly due to less frequent ovulation or to other problems such as endometriosis.  Endometriosis is a condition in which tissue similar to that lining the uterus attaches to the ovaries or fallopian tubes and interferes with conception.

If you are over 35 and trying to conceive, you should see a physician after trying for around six months.  Some studies have shown that about one-third of women between age 35 and 39 and two-thirds of women over 40 have fertility problems.  When they do get pregnant, women older than 35 may have a higher risk of developing gestational diabetes, preeclampsia, and placenta previa. There is also a potentially higher risk of having a baby with low birth weight, and of chromosomal abnormalities such as Down syndrome.

Many cases of infertility can be treated successfully.  While women over age 35 may have more difficulty conceiving, they also have a greater chance of bearing twins. The likelihood of naturally conceived (without fertility treatment) twins peaks between ages 35 and 39.