It is not possible to book tickets for an unborn child. Therefore, you should wait until the baby is born and has its own passport before you make the reservation. Future names for babies are not accepted.
We cannot guarantee that babies that have not been booked at the same time as other passengers will be permitted to fly, even if they are going to sit on an adult's lap. This depends on the airline's availability and technical possibilities outside our control. It is also not always technically possible to or correct a name/date of birth of an existing booking.