I was in a really similiar situation..had just been promoted into management, into a position where the previous 2 had left after having their babies. They took a lot of days off due to morning sickness, appointments etc, and the upper management and inferiors didn't look too kindly upon it. I didn't want anyone to judge my performance based on my pregnancy.
Unfortunately not all employers are sympathetic of pregnancy! Even though there are laws to protect us sometimes there are bad apples in the batch and they find ways to make life miserable for you. My boss was a woman with 2 children and make life unbearable for me my last 2 months. She made sure that every loophole she could use she did. You have to protect yourself and your job if you are required to work. I'm glad that I didn't tell earlier. It would have been a lot more stressful for me. In the end you have to make a decision that works with your situation and not every situation warrants the same decision.
Women are insightful! I'm sure they will figure it out, or least they will assume. But you can keep them guessing for a while!