Most women are much more fertile after surgery, even with moderate pre-op weight loss. Birth control pills do NOT work as well in heavy patients. Birth control pills are not very reliable during the time your weight is changing. Menstrual periods can be very irregular, and you can get pregnant when you least expect it!
Most groups recommend waiting 12-18 months after surgery before getting pregnant.
Many women who become pregnant after surgery are several years older than their friends were when having kids. Being older when pregnant does mean possible increased risks of certain problems. Down’s syndrome and spinal deformities are two examples. The good news is that, after surgery, there is much less risk of experiencing problems during pregnancy (gestational diabetes, eclampsia, macrosomia) and during childbirth. There are also fewer miscarriages and stillbirths than in heavy women who have not had surgery and weight loss.