Excuse to leave early to do PIO shot

Our families don’t know we’re doing IVF or even trying to have a baby. My husband’s bday is coming up and we’re having dinner at his parents’ house. We usually stay late playing games and stuff but we need to leave early to do my PIO shot on time. My husband doesn’t want to try to do it there in secret. I usually sit on a heating pad for a while after and I’d be disappeared for no reason for a long time. It’ll just be the 4 of us so it’d be noticeable.

I can’t think of a good excuse to leave early that they won’t ask questions about. They are a very open family so being cryptic would be weird. It’s not on a work day so there’s no reason to get up early.