I would like to discuss the differences between the birth of a baby in humans & the great apes e.g. gorillas, chimpanzees, & orangutans. In this part 1 we will first look the normal birth process. In part 2, I will summarize recent literature comparing the differences between man & the apes. Finally in the part 3 I will tell you about recent fascinating findings on the fossil skull of the Taung child and the insight it offers into the growth & development of the brain before & after birth in human babies
I argued in my first blogs that difficult biological processes can be simplified by the repeated use of familiar analogies. One of my favorites is the flow model using the very simple “milk shake” analogy because it is an activity many enjoy. It is certainly easier to understand & remember than the original “Law of Poiseuille”. You will have to grant me certain liberties to use this law which was developed to describe the flow of liquid. The main variables in the law as simplified in an earlier blog are pumping or suction pressure, diameter of a tube, & viscosity of the liquid (see figure ). The uterine pressure & size of the pelvis are thus easy to accept in my analogy, but I also ask you to consider the baby as a “viscous object” for the sake of keeping it all simple. Perhaps the request is not so strange when we consider that babies’ body content is 80% water!
So when labor begins uterine contractions will increase in frequency & force. The resulting pressure pushes the baby down so that its head will gradually dilate the resisting soft tissue of the cervical opening of the uterus i.e. increase the diameter. The head must also pass the bony winding course of the pelvis. The diameters of the various planes in the pelvis can make this a difficult course to pass. This will be discussed in blog part 2. These diameters will yield slightly as a relaxin hormone does loosen the ligaments binding the different pelvic bones together. But now we come to the “viscosity” of the baby. The soft tissues of the baby will compress. But what about the large bony skull? Over millions of years a human fetal skull has developed ”soft spots” i.e. fontanels & sutures which allow the skull to be compressed & the skull bones to slide over each other (see next blogs on Taung child).
Thus normally the birth process proceeds slowly but surely as the baby passes through the pelvis. This, in summary, requires a) adequate uterine pressure, b) a wide enough pelvic space, and c) a baby who is not too large.
However prolonged labor is a common problem. For example the uterine pressure may not be adequate & there may be a mismatch between the size of the baby’s head & the mother’s pelvis. For a more detailed summary of the causes of prolonged labor see this obstetric website.