I will be 35 in June. I am overweight but active, and I have regular cycles. My husband is 36, and we have two healthy sons, age four and seven. We would love to have another child soon. What are the odds of our having a child with birth defects because of my age? Does it make any difference that I did not start my periods until I was 16.5 years old? Do two previous healthy uneventful pregnancies lower the chances of birth defects or chromosomal problems? I have searched the web endlessly and have not found answers to these specific questions.

Kiara

One chief concern of pregnancy after age 35 is increased risk of chromosomal abnormalities. Age-related chromosomal abnormalities such as Down Syndrome occur in only about 1 in 200 pregnancies (about one-half of a percent) for women aged 35, so this is not a great concern for you at this time. However, for women age 40 at the time of pregnancy, the risk rises to about 1-2 percent.

Age-related chromosomal problems typically originate at the time of meiosis, when the egg cell eliminates half of its 46 chromosomes to accommodate the male's genetic contribution. The chromosomes are separated by tiny filaments called spindles, which appear to become brittle and break or to become detached from the chromosomes as women age. This breakage or detachment can result in an abnormal number of chromosomes in the egg, a condition called aneuploidy. This occurs in about 33 percent of eggs at age 35 and 50 percent of eggs at age 40. Luckily, few of these irregular eggs will fertilize or develop into detectable pregnancies. As such, the risk of a genetically abnormal pregnancy is much lower than the risk of an abnormal egg.

As you age, your risk also rises for nonchromosomal birth defects and pregnancy complications such as gestational diabetes, pre-eclampsia and intrauterine growth retardation. At age 35, however, your age plays little role in any of these complications. The age at which you began menstruating (menarche) does not seem to influence this risk, nor does your prior history of a normal pregnancy.

I'm sure you are well aware that there is a significant decrease in fertility as a woman ages, and that now we have testing available to evaluate your fertility potential.