Copyright

Third Trimester Fetal Development

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: Childbirth: The Role of Hormones in Labor and Delivery

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
 Replay
Your next lesson will play in 10 seconds
  • 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
Add to Add to Add to

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Login or Sign up

Timeline
Autoplay
Autoplay
Create an account to start this course today
Try it free for 5 days!
Create An Account
Lesson Transcript
Instructor: Heather Adewale

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

This is it - the last step, the last few months before the birth of a baby! Learn about the changes the developing fetus goes through during the third and final trimester.

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.

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!

To unlock this lesson you must be a Study.com Member.
Create your account

Register for a free trial

Are you a student or a teacher?
I am a teacher

Unlock Your Education

See for yourself why 30 million people use Study.com

Become a Study.com member and start learning now.
Become a Member  Back

Earning College Credit

Did you know… We have over 160 college courses that prepare you to earn credit by exam that is accepted by over 2,000 colleges and universities. You can test out of the first two years of college and save thousands off your degree. Anyone can earn credit-by-exam regardless of age or education level.

To learn more, visit our Earning Credit Page

Transferring credit to the school of your choice

Not sure what college you want to attend yet? Study.com has thousands of articles about every imaginable degree, area of study and career path that can help you find the school that's right for you.

Create an account to start this course today
Try it free for 5 days!
Create An Account
Support