I would agree with Flowery, if you can, finish your exams first. Spend the time ensuring you and your DH are in peak conception health instead and research the future job situation so that you are best prepared.
As you're not even 30, unless you have a specific reason to think you'll have problems conceiving I'd probably wait until after your next June exams, if they are the last set.
Anyway, that's not what you asked
In terms of a fixed term contract, you are entitled to maternity leave, and maternity pay. You can't be discriminated against because of your pregnancy/maternity leave, so if they would otherwise have extended your fixed term contract, they can't change their mind about that because of pregnancy.
However they are not specifically obliged to extend your contract if they otherwise wouldn't have.
My husband and I are 35 and 29 respectively. We have been married for just over 18 months and want to start trying for a baby this summer, as we know many friends who took a long time to conceive and are reluctant to wait. The problem is that I'm in the middle of studying for my accountancy exams, which take place every June and December. My plan is that I'll do my December exams as planned (while I'm - hopefully - pregnant) and then defer the rest until I'm ready. Obviously it's all hypothetical but the last thing I want while I'm trying to conceive is to be stressing about exams, and whether my work will let me take a year off, and whether they'll agree to me working part time when I come back. But at the same time I don't want to wait to start trying as it could take a long time. I'm just really worried that I'll jeopardise my job (I'm on a fixed term contract so I'm not sure what my rights will be in terms of mat leave). Any advice about the legal / employment position would be much appreciated!