My experience is from 12 years ago and I do think things have moved on for women in the work place since, but I will tell my story anyway.
I was a couple of weeks pregnant when I was asked to apply for an internal promotion, the implication being that I had a very good chance of getting it. I thought long and hard, I hadn't got a reason not to apply, it was a good job that I would have enjoyed and been good at but as posters have already said any new job is hard work and being pregnant and then having a new baby is also hard work.
Anyway in the end I went to my prospective new boss and explained (far too early in the pregnancy and in strict confidence) that I didn't feel able to apply.. He was very appreciative that I had been straight with him.
A colleague got the job and it turned out she was also pregnant!
So she had a few months of pregnancy where (IMVHO) she wasn't really interested in the job - FWIW I think she really did want it and intended to do a good job when she took it, but not easy in a new job anyway and if you're feeling rubbish almost impossible. Then she had her paid maternity leave on a higher salary than me.
But, when it came to going back the bosses bent over backwards to keep me and find attractive part-time employment. For some strange reason it was impossible to find anything suitable for her, unless she wanted to return f-t to her new high stress job, which she didn't as a new mother.
As a result of being able to "keep my hand" in part-time while my DC were small my career is now well ahead of hers.
I know you're not allowed to say it but employing women of childbearing age is a pain. Every time an employer feels "stitched up" by someone taking a job only to be on ML for 12 of the first 18mths, they will think twice about employing a woman at all next time.