How does the mother’s body know when to deliver the fetus and what happens to it afterward?
What triggers labor is still not fully understood but certain things are known. It seems that it’s a combination of the mother’s body reacting to what’s happening within her womb and the fetus sending out signals that tell the mother that it’s time for it to leave her for the outside world.
Progesterone, the pregnancy hormone, not only keeps the lining of the uterus going so it can house the placenta but throughout pregnancy also relaxes the muscle of the uterus so it doesn’t contract and push the baby out prematurely. The continued growth of the fetus begins to stretch the uterine muscle to its maximum which makes it more excitable. At the same time the aging placenta starts to have more difficulty providing the fetus with everything it needs and the stress makes the fetus send out more of a hormone called cortisol.
Cortisol is very important for making the fetal lungs develop but it also converts progesterone into estrogen a hormone that increases uterine muscle excitability. The combined effects of maximum uterine stretching, placental fatigue and the associated rise in cortisol causing a reduction in progesterone and rise in estrogen is what seems to make the uterine muscle more likely to contract and bring on labor. Along the way other chemicals, like prostaglandins and oxytocin, are released by the mother’s body to help strengthen labor and soften the cervix for the delivery of the baby.
Once outside the womb, the sudden exposure to a cooler temperature and rise in carbon dioxide in its blood makes the newborn take its first breath. This involves a lot of effort because its fluid filled lungs are collapsed and it takes a lot of energy to break through the high surface tension that naturally wants to keep them that way. The fetal lungs being closed was responsible for making it very difficult for its blood to travel there from the right side of its heart due to the high resistance to blood flow. But now that it’s a newborn baby breathing in air and expanding its lungs this changes everything.
Since its lungs were closed the fetus had two shunts that sent blood from the right side of its heart to the left side and the systemic circulation. Oxygenated blood in the umbilical vein coming from the placenta mixed with deoxygenated blood from the lower part of the fetus on its way back to the right side of its heart. There it was shunted from the right to the left atrium through an opening between them called the foramen ovale. From here the blood went to the left ventricle where it was pumped to the fetal tissues. And the deoxygenated blood from the top part of the fetus went into the right atrium and then the right ventricle and was pumped into the pulmonary arteries. But because the lungs were closed most of this blood was shunted from the pulmonary arteries directly into the aorta through a channel called the ductus arteriosus where it then went to the fetal tissues.
After birth, once the newborn baby begins to breathe in and out, this opens up the lungs and dramatically reduces the resistance of blood flow to them. This situation changes the blood flow dynamic within the newborn heart. As noted above, prior to birth, because the lungs were closed, fetal blood was being shunted from the pulmonary arteries through the ductus arteriosus into the aorta because this was the path of least resistance. But now that the lungs are open and blood is flowing there more easily than it would through the high pressure in the aorta this causes the ductus arteriosus to gradually close over the first few days of life. Also, with so much blood flow going to the lungs of the newborn this means that there’s a lot coming back to the left atrium through the pulmonary veins. This increases the pressure within it and prevents blood from coming into it from the right side of the heart through the foramen ovale and within a few days it closes too.
Finally, due to the set-up described above, the amount of oxygen in the arterial blood of the fetus is only about one-quarter of what it is in the mother. Her blood contains adult hemoglobin which at these low oxygen levels can only carry about 40% of maximum capacity, a level that for her would be incompatible with life. But the fetus manages to stay alive despite these very low oxygen levels by making more hemoglobin than the mother does and in particular a different type called fetal hemoglobin which has a different molecular structure that allows it to hold onto more oxygen.
After birth and over the next few days when the newborn is breathing well and its cardiovascular system closes off the channels that had allowed for the right to left shunting of blood, its oxygen rises to normal levels like the mother’s. At this time the production of fetal hemoglobin is turned off and the production of adult hemoglobin takes over. The exact trigger and mechanism for this change is as yet not fully understood.
Three Questions for Mr. Darwin
- How did life anticipate and from where did the information come to tell the mother’s and the child’s bodies to release the right chemicals to bring on labor at the right time?
- How is it that the newborn baby has enough strength in its respiratory muscles to open its lungs up against the high surface tension that has been there throughout its development?
- How did the body anticipate and from where did the information come to tell the fetus to make fetal hemoglobin while it was inside the womb and switch over to making adult hemoglobin after being born?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2018 Dr. Howard Glicksman. All rights reserved. International copyright secured.
