Figuring money, babies and houses...

So my fiancé and I have started trying but we're also trying to buy a house.

We're basing our affordability on me being out of work and on maternity so don't think we can quite afford a 3 bed, but I'm worried about getting a 2 bed as think we'll grow out of it quite quickly and may not be in work to help our mortgage application on a bigger 3 bed.

What to do?! Stretch ourselves now and stay there longer or a smaller cheaper one and move again in a couple of years?!