I think that the more important question you need to ask is whether your wife's pregnancy will affect your ability to meet your mortgage repayments. If it does, then you should borrow less or continue to rent.
Check out the mortgage arrears forum to see the horrible situation many familes are in who have fallen behind in their mortgage payments.
Bearing in mind what Brendan says about affordability etc, if you are going ahead as planned it is unlikely to matter as you will be signing for draw down with your solicitor and not the bank, probably won't have to go back to bank in person at all.
Babies arent that expensive, it like everything else, you can overspend, spend the needed amount or underspend by buying second hand, Baby & Childrens' Markets are popping up everywhere so there is def a demand.
If you have to go back to the bank your wife can wear a big fluffy cardigan. It is none of their business. She could just be overweight. Plenty of overweight people are mistaken for being pregnant.
You don't need to worry about this on top of buying the house and the pregnancy.
Just a note to sort out your life insurance and house insurance in good time for the drawdown, as life insurance can take longer than expected to arrange.