In the last blog post, I cover how and when pregnancy occurs. But what happens after fertilization and the baby starts to grow? How is your baby developing and what does their growth look like in the womb? In this blog post, I will be going through each week in the first trimester and discussing how your baby is developing physically. We will go through the second and third trimesters in the coming weeks, so stay tuned! 

Week 4

     This is when the fertilized egg [blastocyst] implants into the wall of the uterus. The blastocyst is made up of hundreds of cells, inner and outer layers. The inner layer of cells will develop into the embryo while the outer layer will become the placenta. (1) Once the amniotic sac [basically a bubble of fluid] is formed, the blastocyst becomes an embryo. The baby will be inside the amniotic sac throughout the pregnancy. It is filled with fluid which provides a cushion to protect the baby as it is growing. (3) 

Week 4 Fun Facts 

  • Baby is the size of a poppy seed. (2)
  • Baby is growing fast at this point and is very vulnerable. It is important to avoid anything that could harm the baby including alcohol, tobacco, drugs, hot tubs, saunas, or anything that could cause overheating. (3)


Week 5

     The embryo is now made up of three layers of cells. Each layer is the foundation for all of the baby’s organs and tissues. The neural tube is starting to form from the top layer and from the neural tube the brain, spinal cord, and nerves will develop. Hair, skin, and nails are also formed from the top layer. The middle layer will form into the baby’s heart, muscles, cartilage, and bone. And the inner layer will form into the lungs, intestines, and other organs. (4)  

Week 5 Fun Facts

  • Baby is the size of a sesame seed. (2)
  • Baby looks like a tadpole. (2) 
  • This is when the heart starts to beat and sounds like a galloping horse. (1)

Week 6

     If you have an ultrasound at this point, you will be able to see the baby’s heart fluttering. It is not fully developed yet, but there is activity. Where the eyes and nostrils will be there are now little dark spots. And there are impressions on the side of the head where the ears will form. (6) There are also small sprouts or buds that will form into arms. (1) 

Week 6 Fun Facts

  • Baby’s body is in a C shape. (1)
  • Baby is the size of a lentil. (6) 
  • It looks like the baby has a tail, but this is the backbone and the “tail” will eventually disappear. (6)

Week 7

     This week the baby’s brain, facial features, arms, and legs are growing. Parts of the eyes are starting to form and the brain is now made up of three parts. (7) The hands that started as “buds” last week are now webbed (think duck feet) and there are buds where the legs will form. (1) 

Week 7 Fun Facts

  • Baby is the size of a blueberry. (2)
  • Baby’s brain is gaining 250,000 cells per minute! (7)
  • The “tail” is still there.

Week 8

     The fingers and toes are beginning to form from the webbed hands and feet. The upper lip and nose are formed and the eyes are more clearly identified. (1) There are also breathing tubes from the baby’s throat to the lungs, so their breathing system is starting to form. (8)  

Week 8 Fun Facts

  • Baby is the size of a kidney bean. (2)
  • You won’t be able to feel it, but the baby is starting to move around! (2)

Week 9

     Some of the major developments this week are in the baby’s heart and face. The four chambers of the heart are formed and you can clearly hear the beat on an ultrasound. On the face, eyelids form and 10 buds form in the gums that will eventually be teeth. (9) The baby also now has elbows and knees. (1)

This is also the week when your placenta takes over producing hormones that help the baby grow. The placenta is an organ that is attached to your uterus and connects to the baby through the umbilical cord. It will continue to grow with the baby and provide nutrients throughout the rest of the pregnancy. (9)   

Week 9 Fun Facts 

  • Baby is the size of a grape. (2)
  • Baby has ear lobes. (2)
  • Little toes are visible. (2)

Week 10

     The parts of the eyes that allow for sight are now fully developed and the eyelids can shut over the eyes. They will stay shut until week 27 of the pregnancy. The baby’s head is very round due to its fast-growing brain and some of the new brain activity allows for movement of the baby’s knees and elbows. (10)   

Week 10 Fun Facts

  • Baby is the size of a kumquat. (2)
  • The most critical phase of development is complete! (2)

Week 11

All of the baby’s organs are formed and some are already functioning such as the liver, kidneys, and pancreas. By the end of this week, the baby’s genitals will start forming into their proper anatomy depending if they are a boy or girl. (11) 

Week 11 Fun Facts

  • Baby is the size of a fig. (2)
  • Baby’s head still makes up half of its length. (1)

Week 12

     While you still cannot feel the baby moving, they are very active by week 12. Their reflexes have kicked in and they can open and close their fingers, making a fist. They can also curl their toes and their mouths can make sucking movements. (12)

Week 12 Fun Facts 

  • Baby is the size of a lime. (2)
  • Baby has fingernails. (2)
  • Baby is fully formed! They just have some growing to do.

The first trimester is complete! At this point your baby is fully formed and has all of their body parts, but they still need more time to grow before they can survive outside of the womb. This means the most critical growth period is through and the risk of miscarriage goes way down. In the next post (January 26th), we will cover growth during the second trimester when the baby is able to move around much more and their facial features move to their more permanent locations.

For pictures of what the baby looks like during this trimester, click here.

If you have any questions or concerns, please contact;

Nurturing Center of Lubbock
3303 66th Street
Lubbock, TX 79413
Phone: 806-780-6853 (available 24 hours)
Text: 806-317-4480


Marlee Henn, RN

Marlee Henn, RN


Marlee has been a nurse for 2 years and currently works in a functional medicine clinic and part time in a long term acute care center. She has a passion for natural living, womens health, and education.