I thought you had to be working for someone for 12 months before you could get maternity leave? Or is that just paid maternity leave?

Anyway, I agree that you should print out the email and also save a copy and then approach your boss and tell him you would like to take the job and hope that your being pregnant won't affect that. It is possible that he did think from your last conversation that you didn't intend to take up the offer because you are pregnant and that is why it hasn't been mentioned again.