Asking for maternity leave is different for everyone.
You’ve just found out you’re pregnant. (Don't pass out. Trust me.) There are many details to think about, one of which is how it will affect your job. The first question most women ask is: When do I tell my boss? Right away? Before I start to show? Right as I go into labor?
The answer depends on the person. Some women still subscribe to the belief that they shouldn’t tell a soul they are pregnant until the 12th week. This notion is because a number of early pregnancies end in miscarriages, and they don’t wish to spread bad news. However, in the case of your employer, a miscarriage will likely result in a few days off from work, and if your boss is already aware of your situation she will be understanding.
Still, many women prefer to keep the news to themselves for any number of reasons. Avoiding the inevitable onslaught of birth horror stories from near-strangers, and the deluge of unsolicited advice is one. Point being, keep it to yourself as long as you’re comfortable, at least for the first trimester.
This isn’t always possible. A number of women are so exhausted that they’re caught sleeping at their desks. Morning sickness (an inaccurate term if there ever was one, since most women experience nausea morning, noon, and night) will also affect your work. You’ll be sensitive to smells (Does Roger really have to eat Korean BBQ at his desk?) and irritable due to hormones. If you don’t want your co-workers thinking you’ve gone off the deep end, it might be best to confess. They’ll go out of their way to be accommodating, and can make your first trimester much more pleasant.
If you manage to keep the news to yourself until you’re out of the first trimester, definitely alert your boss by the second trimester. If necessary, she can keep the news to herself, but it’s important that she knows by your third or fourth month so that she can begin thinking of arrangements for your leave.
Sit down with your boss and choose a last day, ideally two weeks (or more) before your due date. If you plan to return to your job, let her know. Some companies are required to hold your job for up to 12 weeks, so find out if you qualify. Designate how much leave you will require. If you’re uncertain, see if your boss is willing to be flexible. If she is hiring a substitute or a temp, this may be a little more difficult, but if other workers are covering for you, flexibility is more likely.
Whatever you decide, don't wait until you're showing and hope the managers simply figure it out. Your transition to and from the workplace will be much easier the sooner they know, and if you aren't planning to come back, they need to know this as well.