Cervical Effacement & Baby Engagement: What Does It Mean?

Question
I am 29 yrs old with 2 kids and one on the way.  My due date is sept 14 2005.  I am 36 & 1/2 wks preganant.  I went to the dr on Fri Aug 19th and was told that I was about 50% effaced and my baby's head was engaged...I am curious if this means I could be having my baby soon or just what exactly it means.  

Answer
Effacement is the thinning of the cervix, almost to the consistency of seran wrap. Engagement means that the baby's head is at the level of the ischial spines. Since you are only 50% effaced and you did not mention whether your cervix is dilated at all, you are not in labor. When (for the third baby) you start to have contractions every 6-8 minutes at the level of your belly button (from the start of one contrction to the start of the next contraction) and each contraction lasts (remains hard) for 45 seconds, and this continues for a full hour, we consider you to be in labor.