I kind of like the 'neatness' of your idea and can see why it's so tempting. 'Hmmmm.... if I get the baby and the PGCE all wrapped up in one year, then I can take maternity leave and come back to a nice family friendly career...'
Except... the PGCE will be the second hardest year you ever have (the NQT year will be the hardest!) adding a pregnancy to that would be madness; primary teaching jobs are very hard to find so you will have that added stress in your first year with your new baby; teaching isn't all that family friendly for the first few years (until you've worked out which corners you can cut!!); presumably you wouldn't be entitled to anything more than statutory maternity allowance so finances might be tight too.
Erm, can you tell I think you are mad?
Only you can decide what will work for you, but if I was you, I would decide which is more important to you at the moment - teaching or children? If it is more important to get your career on track, then do the PGCE and NQT, then have children (you will have more support, more money and more idea what you're letting yourself in for!). If it is important to start your family, then have the baby first, then do the PGCE in a few years time, once you have child care etc sorted.
Whatever you decide, good luck!