The Biology of Pregnancy Part 12: Fetal Weight Increase Is Accelerating

Our journey through the biology of pregnancy has not taken us into the third trimester. By the 28th week, you should receive an injection of RhoGAM, if you have Rh-negative blood. RhoGAM is an antibody that protects an Rh-positive child from attack by your immune system and you can receive it a little earlier than the 28th week too. Also by the 28th gestational week, your fetus has reached 1 kilogram (2.2 pounds), and from this point the weight will increase quickly. While getting heavier, the fetus is also thumb sucking, touching its toes, clasping hands, and making fists. A regular sleep pattern has set in, complete with a REM stage, a period of dreaming. Your child looks just about the way a newborn should look, but not as large.

Meanwhile, the brain is under rapid development with an increasing number of folds (convolutions) in the cerebral cortex, and an increasing amount of asymmetry between the left and right brain hemispheres. Fetal muscle tone is increasing. Meanwhile, the air sacs in the fetal lungs are producing increasing amounts of surfactant, the soapy substance that’s vital for breathing outside the womb. The quantity of surfactant, and the chemical properties of the surfactant, are not quite ready for the outside world. But in the event of an unavoidable premature delivery, doctors can give corticosteroids to speed up lung maturity. This is a big reason why survival after birth at 28 weeks is much, much better than it would be a few weeks earlier.

Since the onset of pregnancy, erythropoiesis, production of red blood cells, has shifted through different sites. In the embryonic period, it occurred in the yolk sac. For most of the fetal period, the liver, and to some extent the spleen, have been the main sites of red blood cell production. But during the early third trimester erythropoiesis is moving to the bone marrow, where it occurs in a mature human.

By week 29, the length for the fetus has typically reached 39 centimeters with a weight of about 1,150 grams. A lot of the growth in this phase of pregnancy is bulking up, getting heavier. For the past several weeks, a layer of fat has been accumulating under the skin and around internal organs. As we noted in our discussion a few posts back, there are two kinds of fat: brown and white. Until now, most of the fat being deposited has been brown fat. This is a special kind of fat whose function is to generate heat. Animals that hibernate use brown fat to keep the heart slightly warmer than the rest of the body as the body cools. Newborn humans also have a good supply of brown fat, but the amount decreases through early childhood. At this point in pregnancy, deposition of fat is accelerating, so your fetus is getting plumper. But now it’s the other kind of fat, white fat. Unlike brown fat, which consumes calories to generate heat, white fat is passive. It only stores calories. Think of it as an energy reserve, but it also has some other functions. Although it doesn’t make its own heat, white fat insulates to help keep heat inside the body. It also provides a physical cushion around the body and around internal organs.

Meanwhile, the fetal head keeps growing. Early in pregnancy, the head was very large compared with the body, but then the body caught up. But now, it’s the head’s turn to grow quickly again. The cranium, the curvy bone structure at the top of the head, must expand to accommodate the growing brain. The cranial bones have become increasingly ossified, meaning that they are hardening. But separations between the cranial bones, called sutures, allow for the cranial volume to keep getting bigger. This happens from now until well after birth. In fact, the sutures between the cranial bones will be some of the last boney areas to harden and seal up. This is why, after birth, your infant’s skull will feature two fontanelles, soft areas between the bones that you can palpate with your finger.

As for the mother, the hormones and growing uterus can make her very uncomfortable during the third trimester, when heartburn and indigestion are common, as are hemorrhoids, leg varicose veins, leg cramps, and swollen feet. A lot of the gastrointestinal symptoms happen as a result of pressure that your growing womb exerts against all the contents of your abdominal cavity. Your obstetrician can advise you on how to minimize these symptoms.

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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