Actually, extreme obesity can cause complications with conception AND during the pregnancy/delivery. However the same can be said for women who are underweight. It is important to maintain a healthy weight both for yourself and the health of any future children.
Being overweight or underweight can cause amenorrhea, or the absence of ones menstrual cycle. This can cause infertility, however everyone is unique. That being said it is better to try to stay within a optimal weight range for full health being its of both mother and child.
Sometimes being overweight can impact your fertility. That being said, that doesn't mean you will have a problem or you should not use bc if you are having sex. Only do that if you can raise a child. Check with a GYN to find out if you will/do have fertility issues.