What is major threat to fertility at 35?

Egg quality. Fertility decreases progressively with age in women and, in general, falls faster after age 35 than in youger women, primarily because of a decrease in the number and quality of the egg supply. In addition, other disorders that can adversely affect fertility, such as uterine fibroids, are more common in older than in younger women.
Fertility. The major threat to fertility as a woman ages in the loss of responsiveness of the ovaries to the cues from the pituitary. This can be assessed by testing fsh/lh at cycle day 2 or 3, and mih levels. These tests will give you an assessment of a woman's ovarian reserve ie. The chances to ovulate and conceive.