Arguments and logic won't work here. I think (personal opinion) that both parents need to want the baby. Otherwise the relationship between parents (and parent-child possibly) will be strained and who knows what could that cause in the long run. So you have to, literally - have to - reach a consensus.

I suggest trying to approach the problem from the other side - try to find a house for your larger family, save some money, get a larger car and so on.

You are very young - you still have time. And a large age difference (say 7 years) will mean that the eldest child can be helpful in hcores and possibly take care of the other babies. Consider it.