Second Trimester Fetal Development

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  • 0:07 1st Trimester Review
  • 1:45 2nd Trimester Overview
  • 2:20 Weeks 13 -17
  • 3:51 Weeks 18 - 22
  • 5:47 Weeks 23 -26
  • 7:07 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

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

The growth and development of a baby is an amazingly detailed and complex process. It starts with a ball of cells, which grows all the way to a full grown, eight pound baby in nine months! In this lesson, learn about what happens during the second trimester of pregnancy.

First Trimester Review

Ahhh, pregnancy. That wonderful, magical time when a new life is being formed. People say there's nothing like the glow of a pregnant woman! Of course, underneath that glow there's the morning sickness, the swollen ankles and the cravings. And we can't forget about the mood swings!! But what's a few months of side effects when you consider the amazing life growing inside?

During the first trimester, the embryo organizes into three germ layers
First Trimester

We all know that pregnancy takes nine months and that those nine months are divided into three trimesters. So, let's take a moment to look past all those side effects of pregnancy and take a tour into what's going on inside the mother's womb. In this lesson, we are going to focus on the second of the three trimesters, but first, let's go through a brief review of what happened during trimester one.

Each trimester is about three months long, hence the term trimester. During the first three months, our embryo organized itself into three different cell layers, also called germ layers: the outer ectoderm, middle mesoderm and the inner endoderm. From there, each germ layer will form different parts of the body, like the skin, the muscles and the internal organs.

Once cell organization has been completed, each layer begins to form specialized tissues and organs. This is the beginning of organogenesis. Organogenesis is the process of organ formation. It begins during the first trimester and is continued during the second trimester.

Second Trimester: Months 4 To 6

Okay, so now that we have had a brief review of the first trimester, let's focus on what happens during the second trimester. The second trimester covers months four to six, or weeks 13 to 26. During this time, the fetus will grow in size as its tissues and organs both grow in size and complexity. Starting out about the size of a peach at week 13 and weighing less than an ounce, the fetus will grow to about two pounds by the end of the second trimester.

Fetal Timeline: Weeks 13 To 17

At month four, the fetus can begin to make facial expressions
Facial Expressions

Month 4: By this time our tiny little fetus already has his or her very own fingerprints and their teeth begin to develop. Their facial features are more distinct, and those chubby little baby cheeks everyone loves to pinch have filled out. They can even start to make facial expressions! Their brains have continued to develop and the four cerebral lobes are now present.

By the end of month four, the fetal endocrine system has started producing most of the hormones an adult produces. This is also when the mother starts to sense fetal movements, which will continue to get stronger as time goes on. And the best part about month four? The reproductive organs are developed. Yep! That means you can find out the sex of the baby!

And even more good news! The beginning of the second trimester is when those horribly inconvenient morning sickness and nausea symptoms usually begin to fade, and some of those first trimester mood swings start to disappear.

Of course, with the fading of one symptom, others start to appear, like the appearance of strange food cravings. You know, now that you're not nauseous anymore. And those mood swings, well they tend to come back in full during the third trimester.

Wow, month four is just full of all sorts of changes, so what about month five?

Fetal Timeline: Weeks 18 To 22

Month 5: Well, by now most of our basic development is done. The organs have been formed, the heart started beating back in trimester one, and the brain is already showing signs of activity. What's left now is to refine all of these processes.

What does that mean? Well, for example, the eyes developed back in the first trimester, but there is lot more to eye development than just the growth of an eyeball, right? So, during the fifth month, the retina develops distinct layers that are needed for seeing, but the fetus can't see just yet. Similar idea with the brain - we know the brain has already formed and taken shape, but now it starts to grow in complexity. Neurons continue to differentiate, taking on different jobs within the brain, and signaling patterns will increase.

Hair begins to grow on the scalp by the end of month five
Hair Growth Begins

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