does anyone know if one can develop an obsession about being pregnant? idk if it’s a common subject but you fear that you could be pregnant when you’re not? would this be considered health ocd?

Yes definitely has happened to me before. I coupled it with writing pages of prayer asking god to forgive me and make me not pregnant. The worst thing about it is that you can induce certain symptoms of pregnancy by over stressing like this (for example, excessive stress can delay your period from coming on time)

I definitely had this one when I was younger and had irregular periods. Like, even if I knew for sure that it was an impossibility.

yup. You can literally have any form of obsession, even about what color you like. The only requirements are to cause you a lot of anxiety that makes you want to do compulsions that only worsen it.