Third Trimester Fetal Development

Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

In the last three months of gestation mark periods of rapid growth for fetuses as their organs and senses mature and refine. Explore the third trimester of fetal development: the seventh, eighth and ninth months of pregnancy. Updated: 08/24/2021

Third Trimester: Months 7 - 9

Ahh, here we are - the final round! The last trimester; the end is just around the bend, or the corner. During this last stretch, our mother is probably sleeping in a recliner, has swollen ankles and might be just a tad bit moody or sensitive.

These last few months can start to get a little uncomfortable as our fetus doubles, or even triples, in size. Your baby will grow from the size of a cauliflower to about the size of an eggplant to about the size of a watermelon, increasing from an average of 2 pounds at the end of the second trimester to anywhere between 5 to 9 pounds by the end of the third trimester.

As our fetus enters this third and final trimester, it is considered to be viable. That means that if it was born prematurely, he or she has a high chance of survival.

That's because the majority of organ formation has been completed by the beginning of the third trimester. Now, you may be thinking, 'Then why do we need the third trimester?' Well, even though most of the organs are developed, the third trimester helps to mature them, readying them for the world outside of the mother. It also gives the bones and muscles time to get larger and stronger.

Growth and development in the seventh month
Third Trimester Month 7

Let's take a brief look at some of the main events that occur during the third trimester. First up: month 7.

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  • 0:07 3rd Trimester: Months 7 - 9
  • 1:40 Month 7: Weeks 27 - 30
  • 2:55 Month 8: Weeks 31 - 35
  • 3:47 Month 9: Weeks 36 - 40/42
  • 5:09 Lesson Summary
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Month 7: Weeks 27 - 30

Month 7: During this month, our previously wrinkly fetus begins to add fat layers and plump out, and he or she begins to see! Okay, maybe not in the way that you and I see, but the eyes (which now even have eyelashes) begin to open. And, as month 7 ends and month 8 begins, the photoreceptors in the eyes will begin to distinguish light from dark. This ability to see will continue to develop even after the baby has been born, as they gain the ability to see colors and learn to focus.

Similarly, the brain also continues to develop, adding more connections between neurons and removing connections that aren't being used. This process will also continue after the baby is born, as they learn things like language and take in the world around them.

Also, while our fetus began to hear sounds during the second trimester, it can now start to differentiate high and low pitches of sounds.

As we progress into month 8 (weeks 31 - 35) these changes will continue as the eyes, brain, ears and other organs continue to mature.

Month 8: Weeks 31 - 35

Baby development in month eight
Third Trimester Month 8

Month 8: The brain and rest of the nervous system begins to protect itself by forming layers of an insulation-like material, called myelin, around the axons of neurons (those are the cells of the nervous system). This will increase the speed at which neurons can communicate to each other.

Just like the brain continues to mature, the lungs do also. During month 8, the lungs begin to develop air pocket cells called alveoli that will help the baby breathe after birth. Fat also continues to accumulate, and by the end of month 8, the fetus is around the size of a pineapple, though not quite as prickly as a pineapple. And he or she can certainly move around a lot: somersaults, kicking and having a good ol' party inside mom's tummy!

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