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The Biology of Pregnancy Part 17: The Day has Finally Come

Once you begin week 39, you have passed the point after which any birth is a term birth. Even if you give birth today, it will not be a pre-term birth. From this point, most fetal organ systems are operating as they will after birth, although the cardiovascular system is a major exception. Let’s talk about why. Importantly, the lungs are still deflated. They can’t do anything inside the womb, except continue getting read to be functional lungs in the newborn after birth. Gas exchange — oxygen entering the fetus and carbon dioxide leaving— happens by way of the placenta, which exchanges gases via the maternal blood and the maternal lungs. Meanwhile, in the fetus, high pressure persists in the pulmonary blood vessels. These vessels supply the lungs with blood from the pulmonary artery –a very wide blood vessel that carries blood from the heart’s right ventricle.

The pulmonary artery divides into a right and left branch, each of which carries blood toward one lung and then divides into an increasing number of smaller arteries. Small arteries finally divide into capillaries, vessels so narrow that blood cells must travel through in single file. Then, capillaries merge together into larger vessels, which in turn come together to form veins, which keep merging until finally there are just a few veins carrying blood back to the heart. These are the pulmonary veins, of which most people have four –two from each lung. The pulmonary veins deliver blood to the heart’s left atrium. And so, the fetus has developed a circuit that’s ready to carry blood from the right side of the heart to the left. But the lungs are not needed during fetal life and much of the blood entering the heart’s right atrium has not arrived directly from fetal body tissues.

Oxygenated blood has come from the placenta, through the umbilical vein, then, either through the fetal liver or through a vessel called the ductus venosus, into the inferior vena cava, which carries blood to the right atrium. In the right atrium, mixture with blood from fetal tissues, especially from the upper body, removes some oxygen, but there is still enough oxygen to support other fetal tissues as the blood circulates. Recall that we said pressure in the lung vessels is very high during fetal life. During each heartbeat, the high pressure keeps most of the blood in the fetal heart from entering the two branches of the pulmonary artery that supply the lungs. Instead, the blood is shunted through two passageways that connect the right and left circulation.

One shunt, called the foramen ovale, is located between the heart’s right and left atria. Since embryonic life, a septum, a kind of wall has built up to separate what originally was one atrium into two. The same has happened in the ventricular part of the heart, which is why there are now two ventricles, instead of one. But, even at this late point in pregnancy, the septum between the two atria is incomplete. It has a hole with a flap over it. That’s the foramen ovale and it’s one way that blood in the right side of the heart can bypass the fetal lungs. The other way to bypass the lungs –the other shunt– is the ductus arteriosus. It carries blood from the pulmonary artery into the aorta, the thick artery that carries blood from the heart’s left ventricle to the fetal body tissues.

Now, there are two ways that you can give birth. Vaginal birth is how virtually every mother gave birth until just over 100 years ago. Vaginal birth can start up on its own; you can go into spontaneous labor. Or vaginal birth can be induced. If it’s getting late in pregnancy and you’re not in labor yet, and if you have no contraindications to vaginal delivery, you’ll be schedule for an induction. If you’re entering the 40th week, probably you already have induction scheduled for a certain day, this week, or next.

To induce labor, you’ll be a commercial form of oxytocin, the hormone that stimulates the uterine muscles to contract. It is either administered through your intravenous line that is also providing you fluids, or it’s injected into a muscle. The contractions will increase in frequency, just as if labor had started spontaneously, though induced labor often progresses more rapidly. Also, when you come in for induction, the doctor will “break your water”. This means rupturing the amniotic sac, which also helps to get labor going. Often though, the water breaks spontaneously.

During the early phases of your treatment, you’ll get a visit from the anesthesiologist whose job is to make your labor as comfortable as possible. Most mothers choose what’s called epidural anesthesia, which eliminates pain but allows you to stay awake through the birth, though you will feel pressure each time the uterus contracts. To give you an epidural, the anesthesiologist inserts a catheter between two vertebrae of your lower spine. Through this catheter, an anesthetic is delivered into a space under the bone, but above the connective tissue layers that surround the spinal cord. This blocks pain impulses from the pelvis and legs bound for the spinal cord and brain, so you will not feel pain. Because the catheter is left in place, the anesthesiologist will be able to adjust the level of pain block, as you need it.

The other type of birth is surgical birth, also called cesarean section, or C-section for short. This means delivering the child through in incision in the abdominal wall and uterus, thereby avoiding the birth canal. Depending on your circumstances, a C-section might be planned for you from the onset. But it could also be a plan B. Sometimes, a mother tries to give birth vaginally and it simply doesn’t work. Your cervix may not dilate enough. If this happens, your doctor will give you a drug that does the opposite of what oxytocin does. It will stop uterine contractions and keep the fetus inside while you are prepared for surgery.

Don’t worry if this happens. Remind yourself that you are very fortunate to live now, when C-sections are safe and routine. This was not the case for most of human history. 150 years ago, a mother in labor gave birth vaginally –and if that didn’t work, she died. As with a vaginal birth, you can be awake during a C-section, and you will not feel pain.

You can meet your baby as soon as he or she is born. And most hospitals and obstetricians will allow your partner to be present in the operating room with you. Generally, that person will stand next to the anesthesiologist, just outside of the sterile field, and will be able to talk to you!

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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