I guess I have been in the same situation, though my company was not understanding at all. I worked for them for over 2 years before I had my first child. My issue was that I did not have daycare and wanted to return PT until something opened up. By part time, I may just one day off during the week so my mom could get her doctor appointments in. My "boss" told me that it was FT or no time at all. She then told me that she had heard I was not returning anyway so she had an ad out for my replacement. I reminded her very curtly that I was on FMLA and did not give her any written notice of leaving so if she replaces me while I was gone, she would be hearing from my attorney. She quickly changed her tune and asked if I could find something in two weeks. I told I would be looking. Well, after that I decided she was not worth my time or commute to work anymore...I had to drive 45 minutes each way to get to work.
I just called her up after one week and said that I was unable to find daycare and would not be returning. She was fine with that..or so she said. I guess I get good referrals from them..not sure since I have been working out of my home with medical transcription since shortly after that time anyway.
In your case, I am hoping your employer is understanding. I would just be honest and tell them that it would be a great financial strain for you to return to work though you had honestly thought it would work out, but after researching daycare rates versus income, it is not going to be so. Tell them you feel awful about such short notice but offer to come in for a bit to train your replacement shortly after your maternity leave ends.
Good luck!
J.