Everybody's different. If you want your own biological child, it may be later than you think-especially if you smoke. For one woman, 33 may already be too late, while her best friend may be having babies at 43 or older.

Dr. David Keefe, a leader in the field of in vitro fertilization and a staff member at Women & Infants' in Providence, Rhode Island, worries that some women may delay pregnancy too long, sometimes with the blessing of their gynecologist. Gynecologists in general practice don't usually have extensive infertility training. Routinely caring for older pregnant women then leads them to minimize the effects of aging. Keefe points to mid-life reproductive failure in females as a phenomenon of long-lived species- an evolutionary survival strategy protecting grandmothers from the risk of childbirth. Male fertility is unaffected by aging.

"It's not what's wrong with you, it's what right with you," he explains. "It was more valuable for grandmothers to be infertile to rear the offspring of their offspring." While Keefe stresses that couples should wait until they are ready emotionally and financially to have a baby, they should be aware that waiting too long could reduce their chances of achieving parenthood. While historic studies point to 42 as the average age of a last conception, most women begin experiencing significant declines in fertility at age 35. While the drop is subtle initially, by age 38, fertility levels begin declining sharply. Then until age 44, individual differences among women are the most varied. By the mid 40s just about everyone is sterile. Genetics accounts for between 30 percent and 50 percent of that variability. If a woman's maternal relatives had late pregnancies, she is more likely to be fertile longer herself, especially if she doesn't smoke, has regular menstrual cycles and takes care of herself physically.

The eggs are the problem. At 22 weeks, a female fetus stops making eggs. At birth 80 percent of the eggs are lost. Eggs die daily, not just through ovulation. By the time a woman is in her late 30s, 99.99 percent of her eggs have died and those remaining are less likely to be healthy. The vast majority of eggs and sperm are genetic failures, too flawed to be viable and committed to die. "You can't affect aging, but you can prevent those things that precociously age you," Keefe advises.

Written by Fruma Efreom