Pregnancy policies are set by each operator, and the remote medical limitations make early disclosure essential. Some operators prohibit boarding at or beyond a stated gestational week. As one documented example, HX currently says a passenger cannot board if they will be 24 weeks pregnant or more at the end of the voyage. That's one company's rule and not an industry standard, so don't apply it to a different ship.
The order that protects you: talk to your obstetric clinician, disclose the pregnancy to the operator, confirm your insurance covers it, and get any required medical documentation, all before you pay anything nonrefundable.
We're not going to tell you that Antarctic travel is safe in pregnancy. That isn't ours to say, and the honest framing is that a ship's clinic stabilises rather than treats, and evacuation from the Peninsula is not quick.
Tell us early and we'll find out the specific cutoff for the departures you're considering before you commit.