Originally Posted by GUWonder

It’s possible.

For BA and AA operated flights booked using BA points, I’ve later called up and had lap children added as separately ticketed lap-child infants.

For AA and BA-issued award tickets for QR flights, I’ve done this for lap children at the airports on the day of travel or before when a city ticketing office isn’t around. So if not able to get it done over the phone, consider the possibility of doing so at the airport. In some — but not all — places the airline ticketing offices at the airports are accessible without needing a same-day ticket to access the ticketing counters/staff at the airports.

If you know the full name and sex of the upcoming baby, you can do the lap child or other infant booking with that name and sex even before the child is born. Any entered birthdate and sex for a lap child in the record can be updated later — but the birthdate and dates of travel should line up with the child appearing to be still not yet 2 years of age on booked date of travel.