I have done 6 infant tickets to accompany F fares in the last year and a half (our daughter is 15 months). This particular nuance varies by carrier, even within alliance. What matters is whose ticket stock the ticket is on. For example we are holding SFO-NRT-HKG using JAL and CX. It is JAL stock, JAL rules apply. We paid 10% of the HIGHEST business class fare because JAL sets a cap and does not charge F pax 10% of the F fare. We are also holding LAX-HKG on CX and so this is CX stock, and we paid 10% of the lowest F class fare at time of payment for the infant ticket.

Another nuance, JAL allows a refund of the infant ticket for 100 yen (yes, you read that correctly). CX charges considerably more - about US$120.

Lots of rules to learn with the infant tickets.

In response to the OP's question about the child being unborn, I have never found a carrier where the infant seats are "capacity controls" - so you certainly can ticket you and your wife now and then just get the infant ticket later. But you will be subject to two downsides: (a) if there are "preferred" bassinet positions on that carrier they may be gone (if to another pax without child, they might be moved to accommodate you, but thsi can be a serious hassle; or they might be gone to another pax with child in which case you lose the bassinet position) and (b) if you are ticketed on the stock of a carrier which charges 10% based on the price at the TIME of ticketing, things could get worse (or, less often, better) in the interim period.

Good luck with it. We've seen a lot of nasty eyes in 6 trips. But our baby is quiet and good and doesn't raise a fuss. We've been to Dubai, Europe twice, Asia twice...all good. Enjoy it! ^