Postponing faculty start date due to covid and Pregnancy?
You should chat with your future department, starting with the department chair. (The views of random people on the internet, by the way, should count for very little!) Based on past experience, asking to defer one's start was not uncommon even pre-Covid. Keep in mind that (i) your department wants you to succeed, and making a case that you're more likely to succeed given x, y, and z is a good thing to do; (ii) it's possible that there are external constraints on the department, like a need for someone, perhaps you, to teach certain courses. Still, it's important to realize that (aside from rare, dysfunctional departments) the department is on your side.
Less important, I will note that personally I would think it better to start sooner, in January, rather than when you're due. Having a new baby is exhausting for the first few months at least, and starting a faculty position at the same time sounds very challenging! (I have two kids, one born before, one after, and zero at the start of my professorship!)