The Biology of Pregnancy Part 6: The Second Trimester

Continuing our series on the biology of pregnancy, let’s move to gestational week 13, the beginning of the second trimester of pregnancy, which runs through week 26. The trimester framework is a scheme that has been popular since the early 1970s, when United States Supreme Court (SCOTUS) Harry Blackmun, who authored the majority opinion in the Roe versus Wade decision of January 1973, used it as the basis for balancing the rights of a pregnant women, the state, and fetal considerations with respect to abortion laws. In the Roe v Wade framework, virtually all of the decision goes to the mother throughout the first trimester. In the second trimester, Roe allowed governments some liberty to regulate abortion, though mostly for the sake of protecting the mother’s health, while in the third trimester, meaning starting in the 27th gestational week, consideration of the fetus could come into play. The use of the trimester system as the basis for abortion rights and regulation ended with the SCOTUS decision on Planned Parenthood of Southeastern Pennsylvania versus Casey in 1992, when it was replaced with fetal viability. This enabled state governments to pass laws restricting abortion at times earlier than the end of the 26th week. This enabled anti-abortion states to chip away at abortion rights, gradually, up to the Dobbs versus Jackson Women’s Health SCOTUS decision of 2022, which overturned Roe v Wade, but the use of the trimester system has continued in medicine for all of this time, and doctors will continue using it from this point onward.

Now at the beginning of trimester 2, the fetus inside you is about 7 centimeters long. The head is now only one third of that length, because the body has grown considerably. Ossification of bones is well underway, hair is growing on the body, and many facial features are well developed, especially the lips and nose. Vocal cords also have developed by this point and male or female genitals are beginning to take shape. You may be able to get a hint at the gender on ultrasound, although the genitals start off looking the same, so it may be better to wait another week or two. On the other hand, you may know the gender already, if you’ve had chorionic villus sampling (CVS), or if you’ve had early amniocentesis. These procedures are performed on mothers who are at risk of genetic or chromosomal anomalies, or neural tube defects, but they also tell you the child’s gender with 100 percent accuracy. If the fetus is a girl, she now has more than 2 million eggs in her developing. A tiny fraction of those will produce your grandchildren, but most of them will be gone before your daughter even starts to menstruate.

This point in pregnancy is notorious for digestive troubles, notably heartburn and indigestion. These symptoms are common and can worsen as pregnancy advances, but eating small, frequent meals can help. This is also the time when budding mothers get strange cravings, and also may develop a temporary dislike for particular foods that they once enjoyed.

By week 14, the fetus is as long as a large tomato, or a small orange. Looking closely into the womb with ultrasound scanning, you can see the fetus using its facial muscles for some rudimentary expressions, like frowning and squinting! A neck is becoming more obvious and the head can extend almost upright. The limbs now appear proportional to the rest of the body, rather than looking too short. These limbs also can move and you may even see the fetus suck its thumb. The thin, furry material that has been growing for a couple of weeks is called lanugo. It helps keep the fetus warm until enough fat grows under the skin.

The central nervous system (CNS) –the brain and spinal cord—also is undergoing rapid development and increased sophistication The emerging cerebral cortex consist of a left and right cerebral hemisphere. The asymmetry in size and geometry between left and right that distinguishes humans from our ape cousins, and that accounts for right and left handedness, is not yet evident as this point. Meanwhile, the liver is producing bile and has taken over erythropoiesis, the production of red blood cells, which previously took place in the yolk sac.

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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