The Family and Medical Leave Act or FMLA is the federal law that makes your maternity leave possible. (FYI: It also applies to other situations where you might need to leave work for a while to take care of yourself or a family member with a serious health problem.)
This law, which applies to you if your company has at least 50 employees in a 75-mile radius and you've been working there at least a year, entitles you to 12 weeks off in a 12-month period. This leave is unpaid, but it secures your position for when you return, and ensures that you continue to be covered under your employer's health insurance plan and other benefits.
Here a few things you might not know about the FMLA:
You can spread it out
Your 12 weeks technically don't have to be taken all at once. If you have pregnancy complications, you might want or need to take some time off before baby arrives. The rest can be taken anytime over the 12-month period that you work out with your employer. Since there are a few different ways to define "12-month period," talk with your HR department about your company's specific policies.
You might have to use your vacation days
Some (but not all) companies ask employees to use paid leave (such as vacation and sick days) that's already been accrued as part of those 12 weeks of maternity leave. You might not be happy about "losing" those days, but look on the bright side—you'll get paid! In fact, the law allows you to elect to use the paid days towards your FMLA leave, even if your employers doesn't require it.
You might have to pay for health insurance
Your employer is required to maintain your health insurance benefits, but you might have to cover your premiums for the time that you're away.
Different states have different laws
The FMLA applies to all states, but most enforce their own family and medical leave laws in addition to the federal ones. Check the National Conference of State Legislatures for your state's specific requirements. Tennessee, for example, offers 16 weeks of leave for childbirth, while some states (California and New Jersey, for example) offer partial wage replacement. Your employer might have varying policies too, so definitely look into your options.
Learn more about FMLA here.