Prenatal care is an important part of your pregnancy, and unfortunately some prenatal appointments may need to take care during working hours. This is even more true if you are opting to take prenatal diagnostic tests like an amniocentesis, which may require you take several days off work. The best approach may be to plan ahead and notify your employer of these appointments as soon as you can. You may need to take annual leave, or you may be legally entitled to go to medical appointments without having to take leave.
In either case, your employer and coworkers will appreciate it if you give them enough time to plan for your absence. How about situations in which you need to accomplish something important while you are also suffering from bad pregnancy signs and symptoms that make you want to run to the toilet to vomit, or take an afternoon nap?
These are, of course, normal parts of pregnancy. Try not to let your performance at work suffer, and make up for any missed work when you are feeling better. Everyone you work with, whether it is your boss or customers, coworkers or anyone else, will highly appreciate your efforts to keep up, even when you are feeling rough. Though, if you are working with heavy objects or substances that can endanger your baby (like fumes), you may be required to drop certain aspects of your work.
This, too, should be discussed with the relevant people as soon as possible. Just like you, as an employee, will need to make some accommodations to make sure you function well at work while you are pregnant, your employer needs to show some flexibility and understanding too. Open and honest communication from all sides is the best way to achieve this.