Regulation may vary depending on each airline, but some airlines refuse to board a pregnant woman whose due date is within 4 weeks from the day of your returning flight.
If this will be the case, please consult your airline and physicians in advance.
Also, depending on airline, infants must be at least 6 weeks old and can be held by adult (a lap child), or use his/her own seat to place it in a reserved seat.
For detailed information, please contact your airline.
*Once a child has reached his/her second birthday, the child is required to have a purchased seat.