The Biology of Pregnancy Part 7: Growth Accelerates

Continuing our series on the biology of pregnancy, let’s move into gestational week 15, when growth of the fetus and womb accelerates noticeably and thus many expecting mothers really begin to show. At this point the length of the fetus is roughly equal to the diameter of a softball, or a grapefruit, plus the fetus is developing some new capabilities. It can sense and react to light. Shine a powerful flashlight on your belly, and the fetus will actually move away from the light. The fetus also can detect sound though her developing ears. The skin is still quite translucent, so blood vessels can be seen throughout the fetal body.

The fetal kidneys are now very active, producing significant amounts of urine into the surrounding amniotic fluid. This is increasing the volume of the amniotic fluid, and so it is much safer for an obstetrician to get a needle into the fluid than it was a few weeks earlier.  Consequently, this is the usual time for having amniocentesis to check a fetus for genetic, chromosomal, and neural tube defects, although it may be performed any time over the next few weeks. Recall that chorionic villus sampling (CVS) is a test that can be performed a little earlier.

If you’re an older mother, which in obstetrics means above 35 years, you’re a candidate for amniocentesis. You’re also a candidate if your doctor determines that you carry a risk for a neural tube defect. Recall that the neural tube is a long segment of tissue that has folded into a tube during the embryonic period and gives rise to the central nervous system –the brain and spinal cord. But there can be imperfections in the closure, resulting in part of the brain or spinal cord uncovered by bone and other tissues that normally surround it. One example is spina bifida, absence of bone around parts of the spinal cord. Another neural tube defect is anencephaly, where part of the skull is missing, along with most of the brain.

If the neural tube has not completely closed, a protein called alpha-fetoprotein (AFP) leaks into the amniotic fluid. This means that the level of AFP in the amniotic fluid is especially high. That’s something that your obstetrician can detect on amniocentesis, but not with the procedure that can be done many weeks earlier to reveal genetic and chromosomal anomalies –namely chorionic villus sampling (CVS). Both CVS and amniocentesis carry some risk, but technological advances have reduced that risk to a very minimal level over the past generation. What this all comes down to is that amniocentesis can provide more information that CVS. But the downside is that you have to wait longer to get amniocentesis.

By week 16, the fetus is about 12 centimeters long, so it could fit in your hand. The skin is still thin and thus mostly translucent layer, so blood vessels and bones still show. But the fetus is getting stronger by the day. It can move the head, neck, and trunk with respectable force, because the core muscles are all getting stronger. The eyelids are closed, but the eyes have moved closer together, rather than being on the sides. Eyelashes and eyebrows are becoming visible. The ears also are moving toward their final position and looking a lot more human. Continuing development of facial muscles and their nervous control is providing the fetus with increasing capability for facial control. The fetal capability for taste and smell continues to improve, and by this time she can hear voices of people talking outside the womb.

The volume of blood in a fetus has been increasing substantially. By this point, the blood volume is about 18.5 milliliters, and the heart is pumping that blood at a rate of about 150 beats per minute. The mother is making more blood too. By this point, you have 50 percent more than the usual amount of blood.

By 17 weeks gestation, the size of the torso and limbs look much more proportional to the head, almost as they will look at birth. Adipose cells beneath the skin are now storing fat, in preparation to provide insulation against cold after birth and to store energy. Also in the skin, sweat glands have developed to help cool the newborn. The placenta, which exchanges breathing gases, nutrients, and waste between the maternal and fetal blood, has grown substantially too. It’s just about the size of the fetus and it’s teaming with blood vessels. As you approach the later stages of pregnancy, your obstetrician will be looking carefully at the placenta’s position, making sure that it does not block the birth canal, which is called placenta previa.

Fetal muscular contraction is getting stronger, so this is also the typical time of quickening, when the mother can first feel the fetal kicks. It can happen a little earlier for some, and later for others. If this is your first pregnancy, you may not feel it for another 1-3 weeks. But once you do start feeling those kicks, you’ll notice them more and more as the pregnancy progresses. The fetus is not breathing yet. The lungs actually are collapsed. The pressure inside them is very high and it will remain that way for the rest of pregnancy. But the fetus is inhaling and exhaling amniotic fluid, as if practicing to breathe.

Since the fetus is now growing faster, your belly is growing faster, producing stretch marks. You’re getting heavier, gaining almost a kilo (2.2 pounds) per week. You also may experience a vaginal discharge and increase sweating. This is partly because of hormones, and partly because your blood volume has expanded.

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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