feeling bad about leaving my job

I've decided to leave my job when I'm 36 weeks. That's only 5 weeks away. I don't get paid leave since I'm part time so I figured I might as well just resign and take as much time as I need with baby and I want some time to focus on my body before he's born. However, I told my employer that I was planning on working up until 39 weeks and I'd be returning after 12 weeks. So now I'll be telling her something completely different. I'm worried about her reaction. I figure if I give them enough notice it should be fine but I don't want them to give me a bad reference when I do decide to go back to work. I work retail management and it's starting to be really hard on my body. I'm not going to push myself if I don't need to.