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The Biology of Pregnancy Part 9: You and Your Fetus are Getting Bigger

Our discussion of the biology of pregnancy has passed the midpoint, at least counting from the last menstrual period, which begins the 40 weeks of gestation. By this point, if genetic testing or testing of fetal tissues has been required and/or if there is concern about  possible neural tube defects, you have undergone either amniocentesis or chorionic villus sampling (CVS). You also will have had a triple test or a quadruple test. We are in week 21, when both your body and your fetus start to get very big. The fetus has reached an important growth milestone. Until now, she or he has been smaller than the placenta that is attached through the umbilical cord. Now, the fetus and placenta are the same size, and soon the fetus will be larger of the placenta. The fetus is approximately 26-27 centimeters, about the length of a long cucumber or carrot. You have been feeling kicks now for a while, but now you may also feel some hiccups, from you, but also from the fetus. It may happen a few times per day and it’s normal.

The fetus has taste buds on on the tongue. By this point, those taste buds are developed enough that some scientists think that the fetus might “taste” remnants of your food that get into the amniotic fluid. But that doesn’t mean that what you eat affects the child’s palate later on. What tasting does do is to encourage the fetus to swallow fluid, which is good practice for drinking and eating after birth. Under the skin, fat, especially brown fat, continues to accumulate, which will help keep the future baby warm.

The brain is developing at very high speed. At this point, it has a cerebral cortex –the part of the brain that does the thinking and the feeling. It does not yet provide conscious, however. The fetus is not self-aware, not sentient. There is a sleep cycle, however. At certain times, the fetal brain at this point shows electrical activity characteristic of being awake. At other times, the electrical activity is characteristic of asleep. Meanwhile, folds, called convolutions are beginning to form in the outer surface of the cerebral cortex. For the remainder of pregnancy, the amount of convolutions in the cortex will increase substantially. This will enable more surface area of nervous tissue to fit inside the cranium of the tiny skull.

Meanwhile, touch receptors on the finger tips have developed so the fetus can explore the uterine environment with its hands. The fetus may tumbling inside quite a bit at this point in pregnancy. Hearing capability has reached a point that the fetus can hear your breathing, your heart beat, and the movement of blood through large vessels. If you’re having a boy, this is the time when his testes begin descending from the pelvis into the scrotum. The testes secrete male hormones, while other glands secreting their own hormones. The pancreas, for example, secretes insulin and is already doing this by this point in development.

By week gestational 23, most organ systems in the fetus are developed enough to support life outside of the womb. Thus, it’s possible for pre-term neonates of this age to survive, but the survival rate is less than 60 percent. A lot of the reason has to do with the respiratory system. Without intervention, the lungs still don’t make enough of a soapy secretion called surfactant, which enables the lungs to expand easily to hold air. Without surfactant, a fetus is not viable. Nevertheless, if tests show that a baby will be born prematurely, or if a pregnancy must end early because of a maternal health issue, such as preeclampsia, obstetricians can accelerate surfactant production by giving the mother corticosteroids. Then, when the premature infant is born, doctors can spray surfactant into the neonate’s lungs to increase the amounts. The newborn can then survive on a mechanical ventilator, until the lungs are mature enough to support life.

Because, survival outside the womb is possible with these interventions, at 23 weeks a fetus is considered to be in a sort of borderline viability. Survival at gestational ages younger than this is possible. The record is 21 weeks and 5 days. But this has happened only a few times. From week 23, though, survival becomes much more common.

As with the lungs, the central nervous system –the brain and spinal cord– is developing rapidly, yet is not what we would call mature. While electrical activity from the cerebral cortex continues producing brain waves, an increasing number of synapses –connections between nerve cells—are forming throughout the brain. Meanwhile, beginning in the brain stem around week 21, a fat-like material called myelin has been forming sheaths around axons, the long appendages of nerve cells. When myelination is complete, electrical signals will be able to propagate much more rapidly. In the neurologic disease called multiple sclerosis, nervous signal propagation deteriorates, because of damage to myelin sheaths. By week 23, myelination has spread to numerous brain sites and it will continue to spread through the central nervous system until well after birth.

By this point in pregnancy, the mother’s breasts may have started producing colostrum, an early kind of milk, which may sometimes leak out.

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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