While there's no specific law covering discrimination against employees due to family responsibilities, there are laws that may protect you if your employer is big enough.
1. Sick/disabled family members
If an immediate family member (or you) has a medical condition that requires regular doctor's appointments, you may well be entitled to up to 12 weeks a year of unpaid leave. This leave can be intermittent, which means that you get up to about 60 days a year or 480 hours. (That's a whole lot of doctor's appointments.)
This applies only if you need to miss work for a serious medical condition of a family member, AND your employer has at least 50 employees, AND you've been there at least a year.
If you know you will need this type of leave, make sure that you notify HR in advance so you make sure you're covered. If you think you qualify and they claim you don't, then contact an employment lawyer in your state to discuss your rights.
2. Sex stereotyping
There's no law making sexual stereotyping illegal, but it may fall within sex discrimination if your employer has 15 or more employees. For instance, the boss who assumes that, because you are pregnant, you won't want to come back to work, is engaging in sex discrimination. Men who are primary custodians of kids, but who don't get the same leave time and schedule flexibility as women, may have sex discrimination claims.
If the boss assumes that you, as a woman, will be the primary caregiver when you adopt and wont be able to travel, that may be discrimination. Men who take paternity leave also can't be treated differently than women who take maternity leave.
3. Family and medical leave
For a serious health condition or pregnancy of yourself or a family member, you may be able to get up to 12 weeks of unpaid leave. This applies only if your company has at least 50 employees, and you've been there at least a year.
The leave can be intermittent, as I discussed above, or continuous. If you take twelve weeks and one day of leave, you're no longer protected. If you are protected under FMLA, you have to be restored to the same or equivalent position.
Even if you exceeded your 12 weeks, the employer can't use your leave to deny you an open position when you're ready to return. If you're pregnant, find out if you're covered or will be covered under FMLA.
An employer can't fire, discipline, refuse to hire or promote you because you're pregnant, because of assumptions about the pregnancy and what you'll be able to do, or because of childbirth. This applies only if your employer has 15 or more employees.
If you need light duty, it gets trickier. They have to give light duty only if they give it for any other temporarily disabled employee.
Company insurance must cover pregnancy, and benefits for leaves must be the same as non-pregnancy leaves. Although you may want to wait until you're showing before you tell management, if you have morning sickness, you'd probably should make that known so they don't accuse you of excessive absenteeism. If you qualify for FMLA leave, you should apply for intermittent leave for morning sickness.
Once you return, breast feeding is also protected. If you need to take time to pump, the employer can't discriminate against you for this or deny you breaks to pump breast milk.
5. Marital status
Some states (e.g., Florida, Illinois, California, Maryland, and many others) and government employers have laws saying it's illegal to discriminate on the basis of the fact that an employee is married or unmarried. However, most employers still can discriminate against you based on being married to a particular person (such as anti-nepotism policies). If the employer has anti-nepotism policies saying that two spouses can't work together or can't supervise each other, they still can't discriminate based on gender (firing only the female employee, as an example).
You also can't be discriminated against based on your association with someone in a protected category, such as disability, race, religion, or national origin.
Harassment based on any of these categories also is illegal. However, the Supreme Court says that you need to report such harassment if there's a published harassment policy. I suggest doing it in writing so that they can't deny you reported it, and give them a chance to correct the situation. If you fail to report it, you may be forever barred from suing for the harassment.
Once you're demoted, suspended without pay, denied a position or promotion, or terminated, the laws protect you, whether or not you report it to the employer first. If you've been demoted, denied a promotion, denied a position, suspended without pay, or terminated, and you believe it was because of family responsibilities, contact an attorney who handles employment law in your state to discuss your options.