Tell the lady you're about to have an accident

As quietly as you can, you talk to the woman.

"I'm really sorry, could I cut in front of you... I'm really close to having an accident."

The woman looks back at you sympathetically, but shakes her head no. She leans in close and talks back quietly.

"Sorry hun, but I am too. If you don't think you can hold it, there's the door to the alley" she says, pointing down the hall.