Here's what I found on this question: "If you work for a company with 15 or more employees, it is illegal for your employer to discriminate against you because of pregnancy, childbirth, or pregnancy-related conditions." This same source said it is not required for an employer to change the conditions of the job to make it easier for the person who is pregnant. The EEOC website said companies smaller than 50 employees, but that was talking about giving breaks to breastfeed, I didn't find the information about "how small is too small" on the first page of info on their site (which is as far as I looked). I used to work for a company with only 4 employees and it was exempt from almost all of those laws because it could show it would go out of business if it had to do things like save a woman's job for her if she went on maternity leave.
It's your right to tell them or not. It's illegal for them to fire you over it. Unless you are high risk or have certain stipulations you need to follow I would hold off until it becomes noticeable. If anyone asks why you haven't said anything about it I would say because it doesn't affect my work ethic or effort. You are there for one purpose, to make money not friends. I work in a hospital with very infectious diseases. I mentioned it because of ppe precautions my Dr advised me to let my charge nurse know so that she can advise me when I need to be extra careful.
Did you just find out or did you know when you took the job? (I will say, even if you did know, you will get along better with your employer if you pretend it came as a surprise.)
My thought is you don't have to tell anyone yet, wait until your appointment at the earliest, unless you are asked to handle anything that is actually toxic. Pretty soon the doctor will want you to stop lifting anything heavier than 30 lbs., if you are going to need accommodation for that, you should tell by then.
What you can do in the meantime is be a good, helpful, dedicated employee, and work hard. That way, even if you do go when the baby is born, they might give you a chance to come back.