When should I stop working?

Harley
I have a part time job at a Motherhood Maternity store. They're a bit ridiculous when it comes to hours. You have to make your sales goal for the day or you don't get a lot of hours. I understand the logic there but it's ridiculous working a max of 9 hours a week. I guess I just want to know when a good time to stop working is? 
I'm currently 27 weeks. Earlier in my pregnancy I had a threatened miscarriage. I try to take things easy to ensure a healthy pregnancy. For the most part I've had a healthy pregnancy. I just haven't gained more than 10 pounds so I'm a little worried about preterm labor because of this. Sorry first time mom so I'd like to hear opinions.