We are sorry to hear of your miscarriages.
A woman is technically required to observe her onot during the first three months of pregnancy. In practice, however, there are usually no onot to observe. If a woman observed her onot during the first month of pregnancy (often this occurs before she even realizes she is pregnant) and didn't begin bleeding, and does not experience any bleeding during her pregnancy, then she no longer has any onot to calculate for the following month. Whether or not a woman is pregnant, onot are calculated only from a flow similar to a period, not from staining (even if it made her niddah).
If a woman does experience a monthly flow during the first few months of her pregnancy (it is uncommon but can occur), she needs to observe onot during the first three months. Additionally, if a woman has established a veset kavua for a specific day of the Hebrew month, she should observe that date as an onah during the first three months of pregnancy even if she doesn't experience any bleeding during her pregnancy.
We give you a lot of credit for examining your actions and trying to understand why you are going through these difficulties. However, it is important to understand that miscarriages are common and you shouldn't make yourself feel guilty that you are at fault. We hope you are discussing this issue with your doctor to rule out any possible medical cause for the miscarriages. Unfortunately, often there is no specific cause, and it is just nature's way of ending an unhealthy pregnancy.
Please see the end of our article on Miscarriage, as well as our article on the Challenge of Infertility, for links to websites and organizations that offer support for couples experiencing pregnancy loss.
Please feel free to get back to us with any further questions.
B'hatzlacha!