For pregnancy, 35 is considered advanced maternal age (AMA). The vast majority of 35 year old women have normal, uncomplicated pregnancies. 35 is chosen because of a statistical comparison of the risk of down’s syndrome with the risk of amniocentesis. Risks of complications of pregnancy go up very gradually with age. The age related risk of a 35 year old are very similar to a 34 year old but much different than a 25 year old.
The risk of down’s syndrome in a 35 year old women is 1 in 385, the risk for any trisomy (extra chromosome) is 1:192 at age 35. The risk of miscarriage from amniocentesis is 1 in 200. This is why 35 is chosen as advanced maternal age (AMA). 35 is the first age where the risk of a trisomy is greater then the risk of the test that can diagnose the problem. There is not a giant leap in risk from 34 to 35. The risk continues to go up so that the risk of trisomy at 38 is 1 in 102, and the risk of a trisomy at 44 is 1 in 26. These are a lot of statistics, the point is that there is not very much risk of a problem at 35. Even at 38, more than 99% of pregnancies do not have a trisomy.
Because 35 year old women have a greater risk of having a baby with a trisomy than the risk of amniocentesis, this test is offered to women who are 35 years old or greater. This does not mean that you have to have the test or even that you should have the test. It just means that the risk of the problem is greater than the risk of the test. Many women that are AMA choose not to do genetic testing of any kind. For a discussion of genetic testing see this video.
Having a baby with genetic abnormalities is not the only age related risk. As women age, there is a greater chance of developing high blood pressure during pregnancy, gestational diabetes, and miscarriage. These risks rise gradually with age and are relatively small at age 35 but much greater at age 45.
There are other non-specific risks of pregnancy that increase as women age. Pregnancy symptoms tend to get worse, the aches and pains and tiredness of pregnancy increase. The explanation for this is fairly simple: nothing gets easier with age. Our bodies have less recuperative abilities as we get older. It is why a scratch takes much longer to heal when we are 45 than when we are 25. The placentas also tend to not be as resilient as we age. This seems to be especially important after 40. Your doctor may want to monitor pregnancy in a women who is 43 more closely than a women who is 23. This may include tests for fetal well being such as NST as the due date approaches. It is probably less safe to go significantly past the due date in a woman of AMA than a younger woman.
Please keep in mind that these risks are generalities. Even though the risks are increased, the actual problems remain rare. The vast majority of women who are AMA have perfectly normal, uncomplicated pregnancies.