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The Biology of Pregnancy Part 4: Transition to the Fetal Stage

At the start of gestational week 8, the embryo inside you is a centimeter and a half long, about the size of a small grape. It still has a tail, but the tail has shrunk noticeably. The head looks rather square and is the biggest section of the embryo’s body. The limb buds are starting to resemble short arms and legs. Bones of the fingers and toes are developing within webbed structures at the end of each limb bud. You can see the tip of the nose and the beginnings of an upper lip. Eyelids are visible are growing larger. Taste buds are developing in tongue while teeth are forming in the gums.

The umbilical cord is very prominent at this point. It is large compared with the embryo. It contains the embryo’s intestines! The developing gastrointestinal tract is simply too large to fit entirely within the embryo at this point, but it will migrate into the embryo’s abdomen in about two weeks. Week eight is also a time when the embryonic heart is taking more of a human shape. Septation –the division of the atrium and ventricle into left and right chambers– is a gradual process, but a four-chambered heart is emerging. Usually, division of the left and right ventricles is well under way by this time. Septation to divide the left and right atria is nearly complete –but for an opening, the foramen ovale, that will not close until after birth, and for 25 to 30 percent of people it never closes completely. The heart rate is now up to about 150 beats per minute.

By this time, the neural tube has given rise to a concentration of nervous tissue in the head that we can definitely call a brain. Your adult brain represents only 2-3 percent of your total body weight, but for an 8-week embryo, the brain is more than 40 percent of its total weight! The brain is growing rapidly and includes not just the ancient, reptilian brain that controls vital functions, but also parts that evolved more recently in mammals. There is no consciousness yet, but the embryo inside you already has a telencephalon, a portion of which will develop into the cerebral cortex. This is the outer part of the brain that we use to think.

As the brain grows, nerve cells, also called neurons, are forming an increasing number of connections, called synapses. At the same time, long appendages of neurons, called axons, are organizing into bundles called nerve tracts in brain and spinal cord, and nerves in the peripheral nervous system. Formation of nerves and synapses, plus organization of muscle cells into muscles, can produce some subtle movements in the embryo, but you are many weeks away from perceiving any movements yourself (a phenomenon known traditionally as quickening). During the 9th gestational week, the embryo now measures just over 4 centimeters from head to rump. Along with seeing the heart beat on ultrasonography imaging, you also may now hear the heart on what doctors call a Doppler monitor.

Limbs are getting longer. Fingers and toes are continuing to develop within webbed hands and feet. Embryonic eyes are visible and you’ll notice that they are very lateral in the head, kind of like eyes of a fish. Eyelids have grown in and they are closed over the eyes. In the abdomen, the liver is becoming more prominent. The gallbladder and spleen are also visible. Until now, the bladder and urethra –the lower part of the urinary tract– have been part of the digestive tract, but now these organs are starting to separate.

The heart now consists of four chambers that are mostly separated between left and right. Meanwhile, a valve has formed between each atrium and its ventricle –the atrioventricular valves—plus the semilunar valves have grown in. One semilunar valve controls blood flow from the right ventricle into the pulmonary artery. The other semilunar valve controls blood flow from the left ventricle into the aorta, the massive artery that supplies body tissues.

This part of pregnancy and embryology represents the transition from embryo to fetus and the circulation of blood in the fetus is very different from what it will be after birth. Now, and throughout pregnancy, only a small fraction of the blood in the heart passes from the right ventricle into the pulmonary artery. Of the blood that does enter the pulmonary artery, much of it then diverts into the aorta through a passageway called the ductus arteriosus. Only a minute amount of blood actually goes through the lungs, because the lungs are useless until birth, when the baby takes its first breath of air.

At this point in pregnancy, the woman does not look pregnant. The abdomen may feel protruded, because of bloating, but the uterus is not yet very large. Nausea of pregnancy may be pretty uncomfortable by now. Heartburn is also common but eating small meals spread through the day can help.

This also is a time when it becomes possible to test the developing child for genetic and chromosomal abnormalities. Chorionic villus sampling (CVS) is procedure that can obtain cells of the developing child beginning at this point in pregnancy. If you and your partner are at risk for a genetic disease because of your family histories, speak with a genetic counselor, if you haven’t done so already. If you are an older mother, the risk of chromosomal anomalies, like Down syndrome, goes up. In either case, you may be a candidate for CVS.

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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