The Biology of Pregnancy Part 14: Mid-Third Trimester

By week 33 of pregnancy, the fetus has reached about 2,000 grams (4.2 pounds) on average, so it’s in the realm of newborn size. If you’re having twins, there is less space in your womb, so you can expect labor and delivery about 4 weeks early on average, so this is the point in pregnancy when you should be prepared for the onset of labor. If you have a singleton, you still have a lot of time for the fetus to get much bigger. You have more time to choose a name, more time to arrange your crib, bassinet, nursery, more time to purchase baby clothing and diapers, and all the things needed in advance of birth.

At this point colostrum, an early form of breast milk, may be secreted from your nipples with increasing frequency. You can still exercise at this point, but you should be slowing down considerably from what you were doing in the early stages of pregnancy. If you can handle a long walk at this point, that’s wonderful. But keep in mind that your growing belly can make it difficult to walk. Your balance may not be so great right now, so you have to be careful not to fall. Very late into pregnancy, some mothers are perfectly comfortable and quite safe performing certain physical activities that might scare others.

Physical activity does not mean skydiving or scuba diving. Those activities are off-limit to you, until well after you have given birth. But cycling is an example of an exercise that some women will tolerate well into pregnancy while others won’t. Swimming, on the other hand, is acceptable for almost any mother, right up to the end, as long as you’re a safe swimmer, as long as you haven’t been restricted from activity due to your pregnancy being high-risk. Don’t overtax your body. Know your limits, but don’t cower on your sofa either. Moving around a reasonable amount can keep you fit and in better shape for labor.

In week 33, various body systems in the fetus are operating almost as they will after birth. One of those systems is the immune system. There is a barrier in the placenta between maternal and fetal blood vessels, but some products from your blood can cross over. A class of antibody, called IgG can cross the placental barrier, for instance. This provides the fetus with early protection. This includes protection against COVID-19 if you get vaccinated or boosted against SARS-CoV2 (the virus that causes COVID-19) while you are pregnant.

By week 34, the fetus is up to 2,150 grams (4.73 pounds) on average and the volume of amniotic fluid, which reached a peak within the past few weeks, is now decreasing to make room for the growing fetus. Full maturation of the lungs, central nervous system, and also a few details of the heart and great blood vessels requires a few more weeks. But the lungs are boosting their production of 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. For this reason, in the event of premature delivery at 34 weeks, the survival rate is very high.

By week 34, 75 percent of fetuses have tumbled into a vertical lie within the womb. In most cases, this means vertical with the head down, facing the birth canal. The opposite is a breech, meaning rump down. There are different types of breech positions. In many cases, if you have a singleton, it is quite possible to perform a vaginal delivery. However, there are a fair number of obstetricians who would opt for a surgical birth –a cesarean section. The preference various from country to country and also depends on other details of the pregnancy. With twins, the changes of a cesarean will be needed are higher if one twin is in a breech. Sometimes, at this point, it is possible for an obstetrician to rotate a breeched fetus before it gets too big and too low in your womb. You should be drinking lot of water to stay hydrated. You should have everything ready to go to the hospital on short notice.

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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