She needs to make an informed choice. SHE. So tell her the scan (and perhaps some other aspects of treatment, like medication/anesthesia) has risks to a fetus and ask if she could be pregnant, even offer a test if there's a possibility.

And make sure that question is asked and answered in private!

But her cycle timing is not necessarily relevant. It's highly unlikely to get pregnant during your period, but possible since your ovaries alternate and can be out of sync. Meanwhile, someone who hasn't had sperm anywhere near her in six months isn't gonna be pregnant even if it's been three since her LMP.

What would you do if she were unconscious, refuse to treat in case it might harm a hypothetical fetus? You might say that's ridiculous but it's happening in 6-week ban states now. And when women are pregnant and miscarrying they've died of sepsis because there was still a fetal heartbeat.