Like most expectant working mothers, you may have to do some research on your employer's family leave policies, make a few financial and personal compromises, and carefully negotiate your maternity leave to get what you really want.
Talk to your supervisor first. Your planning will go better if you can bring your immediate supervisor on board as an ally. Plan to tell your supervisor about your pregnancy before you tell coworkers; most managers hate hearing things through the grapevine. Telling your supervisor early in your 2nd trimester will let her know that you're willing to work together to do what's best for the company. However, before you approach your boss with your news, talk with friends and colleagues outside of your company about maternity leave arrangements they've been able to negotiate. Decide on one that seems ideal for you and approach your manager with a specific course of action and an open mind.
Be proactive. Anticipate your manager's concerns and be ready to address them as you discuss your own needs. By asking your supervisor to help you form a plan that works best for you, your coworkers, and the company, you're showing company loyalty. As a result your supervisor will have faith that you're still focused on your job. Estimate when you would like to leave work, how long you expect to be gone, and what you think must be done before you go.
Put a priority on communication. Throughout your pregnancy continue to communicate with your supervisor about what steps you're taking to make this transition -- and your absence from the job -- easier on everyone involved. For instance, make a list of the duties you routinely perform each day; this will help your replacement fulfill your regular duties while you're out. In essence, you want to show that you're willing to manage your maternity leave the same way you do your job: with savvy, common sense, and knowledge of what's good for the company. Before leaving, create a folder and a computer file where your replacement can note changes that occur when you're gone. That way you can update yourself faster upon your return. Let your supervisor and other key coworkers know that they can contact you at home in an emergency.