What effect does a 40 week baby have on breathing?

SOB during pregnancy. It is not entirely clear what you are asking, but i will assume that you are talking about your breathing. Women who are pregnant will commonly report that they have trouble "catching their breath". This is called "dyspnea of pregnancy". This is different than shortness of breath (sob). If you are sob (with rapid breathing) then you should contact your doctor. Consider calling your doctor!
May be. It depends on the baby size , position and body surface area . In general 40 weeks sized uterus can affect the breathing activity.