The Biology of Pregnancy Part 16: Beginning Your Final Month

As you enter week 37, about one month remains until your delivery date, provided that you are carrying a singleton pregnancy. If you have twins, your due date is around now and if you have triplets, they probably have been born already a few weeks ago. At this point, the fetus is 2,850 grams (6.3 pounds) on average, and it consists of many trillions of cells

You may experience Braxton-Hicks contractions, or may have experienced them already. These  contractions are not real labor, but they tend to happen with increasing frequency by this point. You’ve got the urinary frequency, vaginal contractions, and all the other things that probably make you want to get that baby out as soon as possible, but delivery is coming soon. If you haven’t been to the obstetrician lately, you should do that now.

In earlier weeks, we have talked about the remarkable and rapid development of the fetal brain. By the 37th week, there’s a respectable amount of convolutions —folding of nervous tissue— in the cerebral cortex. We have talked about myelination –the deposition of the fat-like substance myelin around axons, the long appendages of nerve cells. The so-called white matter of the brain and spinal cord –the areas that transmit electrical impulses long distances– is white because of the myelin sheaths around the axons. Everything else in the central nervous system is grey matter. Myelination is accelerating and spreading as the time for birth approaches.

But another striking thing about the brain is how much it has grown. As in the evolution of our ancestors that took place over millions of years, the size increase in the brain during development of the fetus is staggering, though still much more growth will occur throughout childhood. By age twelve, your child’s brain will be triple the size that it is now, and weigh ten times what it does today as the fetus prepares to enter the world.

Once you complete week 37, if labor begins, birth is not considered premature, even if it’s the first day of week 38, by which time the fetus is gaining around 25-30 grams each day. The average fetus during week 38 is 3,084 grams (6.8 pounds), which would be a good weight, even for a full-term baby, a baby born at 40 weeks. The fetus is continuing to swallow amniotic fluid, and urinating into the surrounding amniotic fluid and excreting meconium from the fetal intestines. Details on the head, such as eyebrows, and eyelashes, are in place, and some hair may be present on the scalp. On the other hand, lanugo, the fetal body hair is shedding quickly, although some will persist on the child’s back. As the lanugo sheds, the fetus also loses the vernix caseosa, the cheesy substance that has been protecting the fetal skin for many weeks. Fingernails and toenails are present by this time. If born today, you could not distinguish your 38-week newborn from a 40-week newborn, except for the weight.

Meanwhile, rapid development continues in the central nervous system –the brain and spinal cord. This will continue well past birth. The brain will grow and become more complex for years. Also, the placenta, the organ that has been nourishing the fetus through the umbilical cord, is beginning a gradual retirement. It’s still active enough to maintain the growing fetus, but its own growth has peaked. It will not get any larger.

The fetal lungs are ready to exchange air with the atmosphere, in the event of birth. Those lungs are making enough surfactant, the soapy substance that enables the lungs to expand. Surfactant contains chemicals called phospholipids and the ratio of the types of those phospholipids in the surfactant is approaching what it should be in the lungs of a newborn infant. After birth, the lungs will be ready to supply the blood with oxygen and remove carbon dioxide. Nevertheless, for the remainder of fetal life, the pattern of blood circulation that sends oxygenated blood from the placenta through the fetal body, largely avoiding the fetal lungs, traveling instead through the foramen ovale and the ductus arteriosus, will persist. We’ll look closely again at that fetal circulation scheme in the next segment, the final segment of this series that will take us to the point of birth. In the meantime, stay hydrated, sleep as well as you can, and have everything ready for that day when you need to go to the delivery room.

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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