During my baby's delivery, I heard my doctor discuss the "0" station. What exactly is that?
When doctors and midwives measure labor's progress, we use our fingers to check inside the vagina for several different characteristics, including dilation and effacement of the cervix, which may be more familiar terms. "Station" refers to how the baby is coming down the birth canal. Dilation, effacement, and station may be checked in the office during a late prenatal visit, or during labor to assess progress.
To check station during a vaginal examination, we feel the presenting part of the baby (usually the head), either through the opening of the cervix or through the thin lower uterine wall. The point on the pelvis that we use as our landmark is the ischial spine, a small protuberance of the pelvic bone that can be palpated through the vaginal wall during internal examination. "0" station is when the top of the baby's head reaches the level of the ischial spine. Most examiners use centimeters above and below the spine as the units for station—from minus 3 which is when the baby is still just floating in the uterus, to 0 when the baby is said to be "engaged" in the pelvis, to plus 5 when the baby has come through the birth canal is "crowning."
As you can imagine, using your fingers to figure out location of the baby's head above or below a small bump on the pelvis is not an exact science, and different examiners may call the same exam slightly differently. But overall the station is a helpful measure of how the baby is coming down through the birth canal.