For air passengers who are pregnant, most airlines require a medical certificate after 28 weeks, confirming that the pregnancy is progressing normally, that there are no complications, along with the expected date of delivery. The medical certificate must confirm fitness to fly (Doctors letter must have been written no earlier than 10 days before the outbound date of travel).
Most airlines do not allow travel after 36 weeks for a single pregnancy & after 32 weeks for multiple pregnancies (e.g.twins). For international flights, the cutoff point often is earlier, sometimes as early as 28 weeks. Please check with the airline you are planning on travelling with.