Can my boss make me work third while pregnant ?


I work about 3 days a week third and 2 first shifts. I told my boss when I started ttc that I would not be working third anymore once I’m pregnant due to the different dangers it could pose on my body and potentially my baby. That was about 3 months ago. This month I saw a few signs of possible pregnancy and I told him that he needed to start finding another night auditor to take over when I do find out. So my question is, can he force me to stay on third shift if I don’t want to ? And if it potentially threatens my baby and her/his health ? Or can he just keep scheduling me third shifts and there’s basically nothing I can do about it? I know most people would quit but I have limited options when it comes to work and I can’t afford to be unemployed right now. Especially if I have a baby on the way.