Fetal Development By Trimester: Timeline of Body Systems

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

With each trimester, fetal development advances in order for it to grow into a health baby that will function and survive after birth. Discover more about fetal development, including the timeline of how fetal body systems develops within the three trimesters. Updated: 10/05/2021

Fetal Development by Trimester

Ah, the miracles of pregnancy and birth. They're a bit complex, they're convoluted, and not everything is understood very well, but we'll do our best in this lesson. The way a child comes to be can be broken down into trimesters, or a period of three months. We'll discuss the important changes the fetus undergoes inside the mother's womb as pregnancy progresses, namely, when important body systems develop.

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  • 0:02 Fetal Development
  • 0:27 First Trimester
  • 2:27 Second Trimester
  • 3:43 Third Trimester
  • 4:36 Lesson Summary
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The First Trimester

The development of a fetus begins right when the woman's egg is fertilized by a man's sperm. At this point, the thing that will become a child is actually called a zygote, but we'll keep things simple for our lesson. After this fertilization occurs, the future child has all of the DNA it needs to develop. Half of the DNA comes from the mom's egg and the other half comes from the man's sperm. The DNA is akin to computer code that drives development. Anyways, during the first trimester, the following cool and important things happen:

The nervous system begins to develop. The nervous system is a collection of cells and organs, such as the brain, spinal cord, and nerves, that send electrochemical signals to stop and start processes within the body. Think of the nervous system as a power plant with wires sending electricity to stop and start appliances within your own home.

The cardiovascular system starts to take shape as well. This includes the heart, blood, and blood vessels. During the first trimester, the heart develops four chambers, blood vessels start to evolve, and blood cells begin to form. The fetal heartbeat can be detected during the first trimester as well. The cardiovascular system is like the pump and pipes that push water around your house to supply your every need, from cooking to showering.

Additionally, the musculoskeletal system evolves. The musculoskeletal system includes the muscles and bones of your body. The skeleton is like the framework of your home, while the muscles are all the drywall, bricks, and what not covering that framework. At first, little structures called limb buds appear. They then turn into the arms and legs, and by the end of the first trimester, the fetus even has little fingers and toes that are visible. The head and trunk of the body also take shape during this time, and joints, places where bones meet, allow for some body movement as well.

Furthermore, the eyes and ears start to form. Other major body organs, including the lungs, stomach, liver, and intestines, start to take shape. External genitals begin to take root.

The Second Trimester

During the second trimester, a fetus continues to develop and grow quite a bit. All facial and body features become recognizable at this point. The fetus can also blink, kick, and grasp like Bruce Lee. By the second trimester, all structures and body organs have been formed, but aren't fully functional, like the kidneys and lungs. The kidneys are the things that help to filter your blood like a filter for your water. Because the kidneys aren't fully functional, the little fetus inside the womb still depends on mom to get rid of harmful substances.

The fetal brain grows even more quickly at this point, the skin develops sweat glands, the nails grow to their full length, and even fingerprints appear so police can stop your baby if it does something wrong! The skeleton becomes more recognizable and the first evidence of layers of fat comes about.

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