What does "90% effaced" mean?

When you're not pregnant your cervix is a long and relatively hard tube.  During pregnancy it should stay that way until it begins to ripen, at which point it softens.  Very late in pregnancy -- which, I would note for the record, is not by any definition at 32 weeks -- the cervix begins to "thin out" or "efface," which means that it widens at the top and basically becomes a wide funnel instead of a tube, allowing the baby's head to drop onto it and apply pressure to it, which advances labor.  Effacement basically means the cervix has stopped being the blocking point for the fetus.  Anyway, 90% effaced means that your body thinks that labor is supposed to begin.

So I'm on 101 headed north to S.F., right by the airport when the phone rings again.  Mom.  "They're saying they might not go to CPMC."  What?!?!  "She's too far along and they're afraid she'll deliver in the ambulance."  [Other voices.]  "Okay, wait.  Don't go to CPMC."  [Other voices.]  "Okay, they're talking about it now."  [Other voices.]  "Okay, they're putting her in the ambulance.  Meet her there."

I make an unsafe lane change to get onto 380.