I didn't get hired because I'm pregnant...help.

Susanna
I need some advice on when to tell a possible employer that i'm pregnant. I had this interview at a retail store a couple months ago when I was very early in pregnancy. I didn't mention I was pregnant, because I was so early. she got the impression from me that I could work pretty much all day every day because I'm not in school and don't have anything going on. That is true, but she was really weird about people taking off and I realized i'll have doctors appointments and won't really feel like working all day every day when I get more pregnant...so the job didn't end up working out. 
I ended up having another interview at another retail place that I was really excited  about. She said constantly how much she had been needing someone and what a great fit I would be and told me she would give me a call when she figures out my hours. We really hit it off! I thought since it wasn't the best option last time not telling the employer that I was pregnant, I should tell this lady.....so I did. And i mean right when i told her that, she started suggesting other places that I could work and she never called me back. So she literally didn't hire me because I was pregnant.
So my question is...do I mention that up front or after I get the job? (I'm 19 years old and 18 weeks pregnant) I feel like you are supposed to be up front but neither option has gone in my favor. It's just frustrating because nothing has worked out and I've been hunting for months.