The Biology of Pregnancy Part 13: Rapid Development of the Fetal Brain, Eyes, and Lungs

By week 30 you are technically 75 percent of the way through pregnancy, but that’s counting from gestational day one, meaning the first day of bleeding of your last menstrual period. So really you are more than 75 percent of the way through. The end is in sight, but things are starting to happen really quickly. In part 12 of this series on the biology of pregnancy, we noted that your infant’s skull is growing rapidly with soft areas and sutures to accommodate the growing brain. This ability of the skull to expand is important in the womb, but also after birth.

The fetus now occupies most of the space within your womb, but the placenta also has grown throughout pregnancy, although the fetus now dominates over the placenta in size. Your obstetrician will be checking the placenta carefully though with ultrasonography. Sometimes, a placenta can be too big or located in a bad position, a position that could interfere with a vaginal birth. At this point in pregnancy, the fetus and the placenta can still shift around. But if the placenta is very low, thus blocking the cervix (placenta previa), your obstetrician will want to watch it closely over the next few weeks.

As time goes on, it will get harder for the fetus to change position. Right now, only about a liter and a half of amniotic fluid remain in the amniotic sac where the fetus is floating, because the fetus is now quite big.

Meanwhile, the fetal eyelids can open and close, and neural connections between the retina, optic nerve, and various parts of the brain are forming rapidly. This is enabling the fetus to see blurry images whenever light enters the womb through the wall of your belly –light does get in, if it’s bright enough. The visual connections also allow for rapid eye movement, which is associated with the dream phase of the fetal sleep cycle. As for the fetal body hair, called lanugo, week 30 is the point when it will begin to fall off, although some of it may persist all the way up to birth.

Meanwhile, the fetus is moving inside you about as much as she or he can move until birth. The fetus is growing stronger. Neuromuscular connections are increasing, but kicking space inside your womb is shrinking as the fetus gets bigger, so you may feel less quickening.

By the time you enter week 31, the retinas of the eyes, and visual pathways in the brain have been under rapid development for a couple of weeks already. The irises now can open and close the fetal pupils, depending on the amount of light. Fetal ears and auditory connections in the brain can pick up and distinguish sounds. Touch, smell, and taste are also well developed. Essentially, all senses are primed for the outside world.

If you’re carrying triplets, you can expect delivery around 32 weeks gestation, but at this point mothers-to-be can experience Braxton Hicks contractions, sometimes called false labor. Should you feel any contractions at this point, contact your obstetrician. You’ll be asked to come into the office or you may be sent to the delivery room. Either way, you’ll be connected to a monitor, which will tell the doctors instantly whether you are actually in labor, or whether it’s merely Braxton Hicks contractions. If it is Braxton Hicks contractions, the staff will try to make you more comfortable. Shifting body positions can help, as can can water or possibly some intravenous fluid, since dehydration is one trigger of Braxton Hicks contractions.

Week 32 is also a time when your uterus is actively preparing for labor and delivery, although, if you have a singleton pregnancy, you still have eights weeks to go in all likelihood. As through the past few weeks, the main activity for the fetus is to gain weight, but the mother is gaining noticeable weight too. 0.5 kilo (1 pound) per week is normal.

The fetal lungs are also now maturing quickly in preparation for birth. They 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 32 weeks, survival is fairly good. In fact, 32 weeks is the normal time for triplets to be born. Even so, the chances of cognitive problem for a neonate born at this point are double what they would be for a full term infant. On the other hand, the risk is half what it was back at 24-25 weeks gestation.

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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