@Grapefruitbreakfast
God, this really annoys me. You absolutely do have the right to go to those maternity appointments in works time. Why should you go on your day off? You would be doing it in works time if you were full time!
As a few posters have said, women get screwed over right from the onset in life. Pregnant women/mothers get screwed over and over constantly, and have been royally screwed over since time began. That's why there's so many new rules and laws for them, and why they're protected.
Not exactly related, but this reminds me of a time some 20 years ago when my employer was trying to get me to go on a course at college that related to my work. (But I didn't really need to do it as I knew my fucking job!) They said the course would make me better at my job. I could do it well as it was. Cheeky bastards. 
They wore the 'investors in people' badge though and wanted to be seen to be training their staff, even if they didn't need it!!! This course was one full day a week every week for about 40 weeks. I worked three days a week. Tuesday, Wednesday, and Thursday. And they picked Friday for me to go on this course, so I'd be going on one of my 2 days off every week for 40 weeks. On a day I was not paid for.
I point blank refused and said 'I am NOT going on a Friday - or a Monday... Monday and Friday are my days off.' And I said 'other people go in works time, so why can't I?' They said 'of course they do because they work five days a week. They have no choice.'
I still said 'no, I am not going. I may only work 3 days a week, and not 5, but I am only PAID for 3 days a week!!! So I either go on one of my PAID work days, or you can make the others go on a Saturday or Sunday, I'm not going on one of my days off.' (Monday or Friday.) Dug my heels in and point blank refused to go.
Tell them to get stuffed. If your appointments are in works time you GO then..... As pps have said, there won't be THAT many appointments anyway FFS.
I reckon the posters saying the OP should 'be flexible' (in other words - do what her employer insists on and screws herself over,) are people who work full time, and don't have any empathy for the part timers being forced to do stuff on their days off, OR they are supervisors/managers in a workplace, or self employed, and don't give a stuff about their staff, as long as the WORKPLACE is OK. 