Have you seen this new study? They are saying that women with the blood type O, which is the most common and almost half the population has, have fewer eggs and the eggs they have are lower quality. Blood type A has the best eggs (fat lot of good that did me!). The study used FSH levels to determine this. FSH hormone levels increase in women with fewer eggs, as the hormone encourages egg production. This is one reason why older women have a higher chance for twins, as high FSH levels mean that more than one egg can be released. Women with blood type O had higher FSH levels than normal for their age group.
You can read more about this study here.
What to think? Obviously more research needs to be done. But if you have blood type O, you may want to look into getting your FSH levels checked!