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The Biology of Pregnancy Part 10: The Late Second Trimester

In our discussion of the biology of pregnancy, we’re now in the last weeks of the second trimester, when the fetus has become noticeably larger than the placenta. In fact, at the onset of week 24, the fetus is nearly 30 centimeters long, the length of a ruler, or baby doll, but it’s only about 0.6 kilo (1.3 pounds), roughly 17 percent the weight of a full-term infant. For the remaining 16 weeks of pregnancy, the fetus will continue to get bigger and heavier. In the meantime, some interesting things are happening. Many fetuses have hair by this early, but the genes that encode the hair pigment have not turned on yet, so the hair is unpigmented, meaning that it’s white.

The same is true of the irises of the eyes. Consisting of muscle tissue whose function is to open and close the pupil, the irises will remain pale, until the pigment genes turn on. Iris color is heritable, meaning it is influenced by a combination of genetic factors from both parents. Genes influence the architecture of collagen fibers that form the structural basis of the iris. This affects how light scatters, which in turn affects iris color. It makes a difference between blue, green, and hazel eyes.

Meanwhile, other genes produce pigments. A gene encoding melanin turns eyes brown, regardless of the collagen fiber architecture. If a child makes a lot of melanin, the irises will be brown. Usually, having one melanin gene from parent with brown eyes is enough to do this, because the dark pigment dominates over lighter pigments. Because production of melanin does not peak until months after birth, and if a newborn’s heredity suggests various potential iris colorings, you won’t know the color until later. If both parents have light blue eyes, you could be sure that the newborns eyes will not turn brown, because such parent’s do not carry, and therefore could not supply, even one gene for strong melanin coloring in the eyes to the child. On the other hand, if one or both parents have brown eyes, the child could end up with either brown eyes, or blue/green/hazel eyes. This is because each parent has at least one gene encoding the needed melanin, but each parent also may have just one such gene and may not have passed that gene to the child. The presence of grandparents, uncles and aunts, and siblings with non-brown eyes can provide important clues.

At 24 weeks, medical authorities in many countries consider a fetus viable –meaning that the chance of survival outside the womb is high enough to justify intervention with life support in the event of premature delivery. In the United States, the observation that neonates could be viable prior to the end of the 26th week (the end of the second trimester) and sometimes during the 23rd week, and on a couple of rare occasions just under 22 weeks, led to a reevaluation of the abortion issue in the Supreme Court of the United States (SCOTUS) in 1992. This happened in the SCOTUS review of the Planned Parenthood of Southeastern Pennsylvania versus Casey decision. This decision upheld the main idea of the 1973 Roe versus Wade decision, the idea that the right to have an abortion fell within the realm of a right to privacy, guaranteed by the Due Process Clause of the 14th Amendment of the US Constitution. This concept of constitutional privacy as applied to abortion and various other issues, such as contraception and same sex marriage, is called substantive due process. However, the 1992 Planned Parenthood v Casey decision also ended the trimester framework as a guideline for abortion rights and replaced it with fetal viability. This allowed anti-abortion states to push back the time after which they could restrict abortion, which led ultimately to the recent Dobbs versus Jackson Women’s Health, which overturned Roe v Wade.

By the 25th gestational week, the fetal heartbeat has dropped to about 130-140 beats per minute, and by the way, you can hear it with a stethoscope at this point. Prior to the advent of ultrasound Doppler scanning, doctors had no way to detect a fetal heartbeat until just about this far into pregnancy.

As for changes in the mother, at this point, her skin may be dry, her nose stuffy, and she may sweat a lot and even feel hot flashes. Pressure from the growing womb can lead to constipation and heartburn. But eating small frequent meals, instead of big meals, can help prevent, or reduce, the latter, while good amounts of dietary fiber and water can help move your bowels. As the womb grows, it also puts pressure on the deep veins in the pelvis. This slows the movement of blood in the deep pelvic veins and also in deep veins of the leg. As the chemical system that allows clot adjusts through the upcoming weeks of pregnancy, these two phenomena together put the woman at risk of deep venous thrombosis (DVT) clots in the deep veins of the leg) and central venous thrombosis (clots in deep pelvic veins). This can lead to a pulmonary embolism, a blood clot in the lungs. For those with a very common situation called a patent foramen ovale, it also can lead to an embolism in the brain, a type of stroke.

In addition to checking you for signs of DVT, during this phase of pregnancy, your doctor will screen you for gestational diabetes, especially if diabetes runs in your family, if you are obese, or if you are older than 30. Screening is very important, because you may experience no 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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