There used to be some trouble is gauging the size of a baby being carried by a large woman, but with the use of transvaginal ultrasounds dating a pregnancy isn't as hard as it used to be. Large women are at higher risk of having babies with neural tube defects, but it still isn't a high risk. It is possible that folic acid supplements will reduce the risks, though there is one study that disagrees with that.
Women who have PCOS may be have a higher statistical chance of miscarriage than the general population. The reason for this increased risk is that many PCOS women have elevated LH, and that kind of hormone imbalance appears to effect some pregnancies. There is research in progress to determine how this can be better managed and lower the risk. Excess weight does not contribute to miscarriage; however, PCOS, which is common in overweight women, does create a higher statistical chance of pregnancy loss than in the general population. The reason is related to hormone imbalance. Both elevated LH and testosterone are linked to miscarriage. There are treatments available that help to minimize these risks.
Some of the tests may be harder to run, but not enough so that you should be frightened. BBWs make higher risk patients for laparoscopy, for example, but it usually can be done by a skilled surgeon. Make sure your doctor is comfortable in doing the surgery on you -- confidence goes a long way!
There is also the potential of having trouble seeking medical care from a qualified reproductive endocrinologist or OB/GYN. Sometimes you have to put up with some rudeness before you find the right place to be treated, but it really should be possible to find a doctor to treat your fertility problem no matter how overweight you are.
You don't! You find yourself a good doctor. It's a pretty horrible feeling when a doctor refuses to treat you because you are large, or suggests you have to lose weight before you will be helped. There is always a smarter, more compassionate doctor out there. What you need to do is not let the monster doctor get to you and find yourself someone better.
You should expect to be treated with respect and tested in the same manner a thinner woman would be. Probably the first things a large woman should be tested for are PCOS and LPD. Check out some of the infertility sites and see what normal testing involves.
There isn't a set way to do this -- no database hiding somewhere -- but here are some suggestions:
Make an appointment and see what happens. If you do this, try to set it up so that you will meet the doctor fully clothed. You don't want to be undressed and feeling vulnerable.
Call and ask to speak with the doctor or one of his nurses. Be blunt and ask about the doctor's feelings toward treating large women. If there is any hesitation or doubt, keep calling around.
Write to all the doctors on your list and explain your situation, including your weight (be honest!). Ask if s/he would be willing to help you find out what is wrong and then help you to get pregnant. Not all of the doctors will respond, but most likely you will find someone who will treat you with respect and really want to help you.