Every airline has ther own policy. The easiest is to ask the flight companny before booking. We flew in the 3rd trimster of pregnancy and had no problems.
The doctor will check for anything that can increase your risk such as diabetis or high bloodpressure. If they find a risk factor they will not sign your letter and you will not be allowed to fly thus wasting the flight ticket. We booked a ticket before getting pregnant so we didn't know but if you know it up front i'd avoid it.