As you approach your due date, your doctor will likely check your cervix for effacement and dilation, but since you can't see your own cervix, it can be tough to really picture what's going on. Maybe this will help:
Think of your cervix as the cardboard tube from a roll of toilet paper. If you look through the end of it into the hole and imagine that hole getting wider, that's what dilation is. Now, if you turn the roll on its side and imagine the whole thing getting shorter, that's effacement. In other words, your cervix gets shorter and shorter in anticipation of baby's birth, until it's paper-thin and ready for delivery. Your doctor will likely tell you how effaced you are in a percentage. (Take out the scissors and cut the toilet paper roll in half, and that's a good representation of being 50 percent effaced!)
Effacement usually starts around the middle to the end of the third trimester, and it's a sign your body is prepping for the birth. But as your doctor tells you how effaced you are, know that it won't predict exactly when baby will be born. Women can have an uneffaced, closed cervix and go into labor that same day, or they can start effacing weeks before labor starts.