It all starts around 40 ladies. I had predictably SHORT cycles...14-18 days, yuck. The around 40-42 they started getting quite unpredictable. Though, I knew I had been in premature ovarian failure since about age 34.
If you're trying to get pregnant, go to your doc and get some blood work done...an FSH will tell you a lot of information, as will estrodial and progesterone levels. Also, since you are 40, they aren't going to make you wait the requisitie 6-12 months of "trying" before they do anything.
Good luck, my dd was conceived, with a lot of help, at 45, and I've a friend who's had 2 over 45, so fingers crossed for you!