My experience is from 10 years ago but I don’t think its changed. I had a staff member who had a terrible pregnancy, she fainted a lot and was generally unwell. She was signed off sick for much of it but was still entitled to full mat leave. PP’s advice about contacting your union rep if you have one, is spot on if work is being difficult. If you don’t have one then joining would be a good idea.
Oh and staff member carried to full term and baby was absolutely fine, a lovely little boy.