Cee Cee's right. If you lose your job over it, take him to the cleaners - there's plenty of precedent for doing it, too! It's just one of those things and they are legally obliged to accomodate you in your temporary state
You don't have to tell him right away, but if you feel like it will make your workload easier to have special considerationn, then it's a good idea to let the boss know about it.
You're not the first and you won't be the last woman to find herself in this sort of situation. The laws exist so that women can stop feeling guilty for being so 'unreliable' as to have children! And, unless the business goes belly up while you're on maternity leave, you have the right to return to work at that same workplace.
You will feel SO relieved when it's all out in the open!