Pregnancy is not a valid reason to punish an employee. Pregnancy is meant to be a terrifyingly wonderful time because you are ushering in a new life, not because you are worried about how your boss will take it. It is unlawful for an employer to harass you due to your pregnancy. Harassment can take many forms from the blatant, like firing you after you announce, to the sinisterly subtle, like demoting you. Don't think that protections for pregnant women begin and end with harassment laws.
Depending upon the benefits that your employer offers, you may be entitled to maternity leave with pay. Under the Pregnancy Disabilities Act, if your employer offers temporary disability leave then your employer must also offer the same benefits to women who are dealing with pregnancy. Basically, an employer is prohibited from singling out pregnant women for special rules or procedures. Everyone must be treated equally and the same.
This is not limited to merely offering disability leave; it extends to any and every accommodation that your boss offers to the permanently and temporarily disabled. This can range from reduced hours, lighter work and even to equipment specialized to assist you. These accommodations are not limited to what is expressly allowed by your boss. It includes anything that is provided informally to a fellow employee. Keep an eye out for these events as they can turn the entirety of your case.
Pregnancy should not negatively impact a woman's career. If you want, you can continue working and may even be entitled to reasonable accommodations. If you are concerned about your legal rights as a pregnant employee, then you may want to consult with an employment law attorney to review your rights. You don't need to fight this out on your own. The right attorney can help clarify your rights and legal position so that you can make the best decision you can for yourself and your expanding family.