DC 2007-05-09, In The Womb 子宫日记 17

It's the final act and the countdown is on. Although it's impossible to tell exactly when the baby will be born, only 5 % of babies are born on their due date, the rest can emerge any time within two weeks of their expected arrival. The mother's kept guessing and waiting for signs, the first contraction of the uterus or the breaking water as the amniotic sac ruptures.

Nobody knows exactly what initiates labor, but it's the baby's lungs together with the placenta that determine the timing. When the lungs are mature, they secrete a protein into the amniotic fluid, which alters the placenta's production of hormones. It slows the release of progesterone and triggers the release of a new hormone, oxytocin, which initiates the contractions of the uterine wall. Oxytocin also inhibits memory and may play a role in helping women to forget the pain of birth and bond with their new babies.

During the first stage of labor, the baby's head is locked in the bottom of the uterus and is bearing down on the cervix, the barrier between the uterus and the vagina. The last thing to pass through the cervix was a tiny sperm 38 weeks ago. Now the cervix must stretch ten centimeters wide to allow the baby's head, her largest part to pass through.

For the mother, the pain of birth can be eased if she delivers standing up, sitting or squatting rather than lying on her back. This can also speed up the first stage of labor and reduce the likelihood of needing medical intervention or a cesarean. Labor's also painful and stressful for the baby, the squashing of the umbilical cord can easily constrict the supply of oxygen. To help the baby cope, her body releases large quantities of adrenalin to keep her heart pumping fast enough. Adrenalin also helps prepare the lungs for the lifetime of work they're about to begin.

Once the cervix has fully opened, the second stage of labor, the actual delivery begins. With each contraction of the uterus, the baby is pushed further through the cervix and vagina until eventually her head is just visible. (Baby's head is just coming here.) The contractions are only minutes apart now and after each one the midwife encourages the mother to push down hard and helps squeeze the baby out. (Breathe now, breathe, breathe. Here's your baby, the baby's head, checking the cord down. Yes, its contractions help this baby out, ok? Here you are. Hello. Honey, it's our typical baby.) As soon as the baby is delivered, her lungs drain off fluid and air rushes in, expanding the air sacs that in an instant begin extracting oxygen to keep the baby alive.