Why Are Other Positions Less Favourable?
When a baby tries to enter his/her mother’s pelvis from any head down position apart from O.A., or head down, back forward between mother’s hip and umbilicus, he is faced with a major obstacle. It can be negotiated, but this usually means a late start to labour, and a very slow birth process.
What is the problem?
First, he will be too upright. He will be facing forward, instead of backwards. Instead of having his back in the spacious abdomen, it will be at an angle toward mother’s spine and her right hip. His shoulder will almost certainly be on the other side of her spine, and has a major obstacle to pass when he tries to turn. This is the sacral prominence— the large bone at the top rear of the pelvis. To move into the pelvis, he must slide between mother’s pubic bone and her sacral prominence. To do this, he must bring his bottom well forward, which is almost impossible until labour contractions tilt the top of mother’s uterus forward.
If he does manage to enter, he must come down in a straight line—should he try to use the sacral curve, he will end up coming face first. This is not a good position to be in.
He is also unable to curl his head onto his chest without bumping into mother’s pubic bone. Therefore, babies in Occipito Posterior seldom begin the journey in an effective position.
What Else is Different?
Firstly, the pressure from “practice” contractions is about 2-3 cms in front of the maternal cervix into the lower uterine segment. Thus, the hormone signals that cause the cervix to soften and shorten are not as effective. This often leads to the so-called “false labours” as the baby tries to move himself into a better position.
Then baby’s attempts are often painful, as he rubs against the pubic bone. These sessions may last several hours, and are very tiring.
If labour is not induced it usually starts with the membranes rupturing, and no contractions for some time. They will start when the uterus has regained it’s “tone”—like a balloon losing its air, the uterine walls lose their tension when the liquor drains away
Labour will be slower than for an O.A. positioned baby, but we will discuss this in a later post.