Excuse to miss work..?

I'm 5 months pregnant and been working 4 days straight this week and only had one day off to do errands before I am set to do another 3 day weekend shift while missing time with my husband. He works from 5am to 5pm weekdays and I work from 12pm to 9pm and have about an hour to talk before we go to sleep and start over again. It's all stressful on me because I have to work exhausted and not get to spend time with my Husband. I don't think they'd let me have Saturday off because of just that reason. So, I need a believable good one. I have just been upgraded to Part time instead of Intermittent and don't want to make a bad impression by letting them be short but having time with my husband is important to me.