Thank you everyone for your help.
Just an update. I have spoken with HR, my boss and my doctor. I have decreased my hours to 6 hour days, 4 days a week with Wednesdays off (as I had a letter for my doctor that I made sure included the specific requirements/conditions I needed). I will be using annual leave and unpaid sick. I have confirmed with HR that in my case (and the company I work for), my hourly rate will not change, my maternity leave will be unaffected. Apparently at my work they "classify me as full time", and by doing this when I go on maternity leave, I "will still have the full time status".
So just letting you know it all worked out fine for me. However if anyone reads this, I strongly suggest you talk to HR and your boss to work out the options before making any decisions.