Fasting for a Pregnant Woman Who Feels Ill

My wife is in her ninth month of pregnancy, week 37. Most of her pregnancy she was suffering from nausea and dizziness. In the last two days, she also had headaches. She is afraid to fast. She will start fasting, but the question is, what symptoms would allow her to break the fast?

Greetings,
She should start fasting. If during the fast she feels dizzy even when lying down, or sees darkness black, or has pains that force her to lie in bed, her status is that of a sick person, and she is exempt from the fast.

Shulchan Aruch, section Orach Chayim, chapter 554