system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When doctors take women’s vitals and basic info at the beginning of an appointment, why do they always ask date of last period? What’s so significant about that specific data?
gpt
Because some medications can kill or cause gross deformities and disabilities in pregnant women's babies that the doctor will be paying for by court order for the rest of his life. 

It's an *enormous* risk not to and it's such an easy question to ask that most women are used to being asked so there's little to no reason not to.

Go look up the court awards for Accutane sometime. Look at those babies head's - watermelon sized, and completely nonfunctional. You want to avoid that.

That said, in most hospitals they are going to go ahead and give you a pregnancy test as a matter of course before they give you a script or treatment, asking about the period is more an easy way to make sure their hormones are functioning normally.