Generally, you have nothing to gain by keeping your pregnancy secret, you disclosing it sooner than later is almost always a good idea. First the employer will find out sooner or later that you are pregnant, so why bother and hide. If your employer is the type of company that doesn't know the law or doesn't mind violating the law then they will fire a pregnant woman whether she discloses her pregnancy right away or not. But you can only bring a claim against them for pregnancy discrimination, if they are on notice of both, your condition and associated medical issues.
If, on the other hand, your employer tries to comply with the law, then the sooner you let them know about your pregnancy and any need for accommodations because of any kind of pregnancy related sickness, the better off you will be, because the employer will have more time to accommodate you by either adjusting your work station, or giving you time off or providing other types of modification to your work, schedule, etc... Having more time to work with you will also make it easier on the employer to do what they need to do to help you.