Workplace Laws

I live in Texas. I've looked everywhere and I can't figure this out, so I'm coming here to see if anyone else has had this delima. I work somewhere that oftentimes has said (bosses and HR have said this) "if you're going to leave early then you can come in early to make up for that." Well, my boss today came in and said I was no longer allowed to do that. My hours are 8-5 and expect to work that. I'm 39 weeks pregnant and was going to continue working until I pop. I have no medical leave left, so when I was leaving for doctor's appointments I would come in early to make up that time to get my 40 hours. 
If, other people in my office are allowed to come in early, leave early, work extra to makeup time is it even LEGAL for him to say I can't?! Has anyone ever ran into a boss who basically forces you to take maternity leave although you aren't ready?