Somebody’s waters break
Definition 1 of 1
Pro Tip 1/3Medical Context
This phrase is typically used in medical or childbirth settings, not casually in conversation. 
When her waters broke, the midwife prepared for delivery.
Pro Tip 2/3Urgency Connotation
Implies an immediate need for action, usually to get to a hospital or call a medical professional. 
Once her waters broke, they knew it was time to head to the hospital.
Pro Tip 3/3Not an Idiom
It's a literal term for a physical event, not a metaphor or figure of speech. 
Sarah was surprised when her waters broke, as it was her first pregnancy.