I called a cousin who works for an airline at an airport, he explained how to do this in a very safe and efficient way.
Infants do not need a seat, airline require a fraction of the price paid by the adult to be paid for the infant (~10%) or sometimes it is free. Hence, just make the reservation for you and your wife and other passengers who need seats, and skip the infant. Once the infant is born and it is the day of the travel, head to the airport a bit early and issue the boarding for the baby on the spot, as I said earlier, airline staff will be happy to issue the boarding because there is no seat required and you can make the payment in the airport. The price should be a fraction of the that specific price you paid. So, no worries at all.
Anyway, to his knowledge this is how it is done in the airline he works for and some other airlines, not sure if it is safe to assume all other airlines do it the same way, but according to him adding an infant who does not require a seat is something easy in many airline systems.