Pregnant? How Your Baby Develops Every Week

So, you're expecting and curious about how your bundle of joy grows while you or your partner are carrying him or her. 

Before we get started let’s explain what the term trimester means. A trimester is 3 months. Simple, right? 

Pregnancies are always divided into 3 trimesters, so 3 sets of 3 months, which equals 9 months. Each trimester is just over 13 weeks. So, technically, your pregnancy should be close to 40 weeks in total. 

Now, we’ve got that confusing terms out of the way, let’s get to what you really want to know. 

Gestational Age

Sorry, another confusing term, which we’ll simplify. This is the way pregnancy time is measured. Still a bit confusing? Yeah, we know, we’ll break it down some more. 

Let’s get the facts straight, most people think that pregnancy is only 9 months, but that’s not when the doctors start calculating from, they start counting from the last menstrual period (LMP), which means pregnancy time is about 10 months, not 9. 

So, how did they get to 10 months? 

Easy, they calculate that the pregnancy cycle started on the day your last period ended, which is what makes it 10 months, not 9. 

Please don’t get this confused with the trimesters because a woman’s body starts getting ready for her baby at the end of each period, so as soon as her period ends, her preparation cycle starts again.

Okay, so now, we’ve covered the absolute basics, we’ll get to the breakdown of how your baby will be developing. 

Week 1 - 2

The first 2 weeks are your menstrual cycle. So, you have your period, and then 2 weeks later, your ovary will release its most mature egg. So now you’re ovulating, remember that this can happen later or earlier, each person is different. 

Now that the egg’s released, it travels down the fallopian tube and towards your uterus. The next step is fertilization, which is when an egg and sperm meet then combine. 

Week 3 - 4

Now, the egg is fertilized, it moves further down the fallopian tube and starts dividing into more cells. It’ll take between 3 - 4 days after fertilization to reach your uterus. This ball of cells floats around in your uterus for 2 - 3 days. The cell ball will then attach itself to your uterus, this is what is known as implantation, and this when your pregnancy officially starts. 

Week 5 - 6

Remember the cell ball that attached to your uterus? Well, in week 6, it turns into an embryo. This starts the embryonic stage which will last around 5 weeks. This is when your baby’s major internal organs are developing. 

Your baby (embryo): 

1. Is less than ⅕ inches long. 
2. Has a basic beating heart, and the circulatory system is still developing. 
3. Has minor development on its arms, legs and a tail (tail bone) that look like buds. 
4. Starts developing the neural tube, which will later create the brain, spinal cord, and other major nerves. 
5. Is developing its umbilical cord. 

Week 7 - 8 

Up until now, the sex organs are still completely the same, neither male nor female. In week 7 or 8, if a gene is triggered to develop the testes, your baby will develop as a male, if the gene isn’t triggered your baby will develop as a female. 

In these weeks your baby: 

1. Is about ¼ to ½ an inch long. 
2. Has formed its heart. 
3. Develops webbed fingers, and its toes begin to develop. 
4. Arms start to bend where the elbows should be. 

Week 9 -10

During the third month of pregnancy, your baby changes from being an embryo to a fetus. This is when your baby’s umbilical cord links to your placenta and uterine wall which will carry nutrients and oxygen to your baby and take away waste. 

Your baby (fetus): 

1. Starts to develop external sex organs. 
2. Is about 1 to 1 ½ inches long. 
3. Loses its tail. 
4. Grows longer fingers and toes. 

Week 10 - 11

Now, your baby will get measured from the top of its head to its bum; this is what’s called the crown-rump length (CRL). And this is when your little bundle of joy will start moving around very lightly. 

Your baby: 

1. Has a CRL of around 2 to 3 inches. 
2. Develops full fingers and toes (they’re not webbed anymore). 
3. Starts getting harder bones. 
4. Starts growing fingernails. 
5. Has hormones that start to develop the external sex organs. 
6. Starts making urine because of its kidney development. 
7. Develops early sweat glands. 
8. Has its eyes fused shut. 

Week 13 - 14 

Week 13 marks the end of your 1st trimester and the start of your 2nd trimester in week 14. Your baby’s gender could be seen through an ultrasound; that is if your baby isn’t shy and hiding away from the ultrasound. If you’re having a baby boy, his prostate gland will develop. If you’re having a baby girl, her ovaries will move from her tummy are to her pelvic area. 

Your baby: 

1. Has a CRL of roughly 3 inches. 
2. Starts growing hair. 
3. Forms the roof of its mouth.

Week 15 - 16 

Now, your baby will be around 4 ½ inches long. If you’re expecting a little girl, she would have formed hundreds of thousands of eggs in her ovaries. 

Week 17 - 18 

You’re now going into you 5th month of pregnancy, and your baby will develop some soft body hair and a slimy coating to protect its skin. 

Your baby has a CRL of roughly 5 ½ to 6 inches.

Week 19 - 20 

Your baby will now be around 6 ½ inches long. Your baby will develop what is known as a lanugo, which is fine hair that covers its body. Its skin will be covered with a vernix caseosa, which is the slimy material that is protecting its skin. 

If you’re having a little girl, her uterus will start forming, too. 

Week 21 - 22

In your 6th month, your baby starts developing its blood cells through its bone marrow, taste buds, eyelashes, eyebrows. 

Your baby has a CRL of 7 inches. 

Week 23 - 24 

Your baby is about 8 inches long and continues to develop its eyebrows and eyelashes. 

Week 25 - 26

Your little one now has a CRL of roughly 9 inches and is developing fat which will continue until your baby greets the world. 

Week 27 - 28 

Week 27 marks the end of your 2nd trimester and week 28 marks the start of your 3rd, and final trimester. 

Your baby has a CRL of 10 inches and still has its eyelids fused shut. 

Week 29 - 30

Your baby is now around 11 inches long, and its fine hair starts to fall off.

If you’re expecting a baby boy, his testes could begin descending into his scrotum, and normally keep dropping and developing until week 40. 

Week 31 - 32

Your little one should now be around 11 inches long, and its fine hair should continue to fall off.

Week 33 - 34

Now you’re 9 months pregnant, your baby is around 12 inches long, and has nearly lost all of its fine hair. Your little one’s eyes are developed enough for its pupils to react to light. 

Week 35 - 36

Your little bundle of joy in now reaching 12.5 inches long, is getting chubbier, and no longer has wrinkled skin. 

Week 37 - 38

You’re now officially 10 months pregnant, remember it got calculated from your last period. Your baby is now in its last development phases before it comes out to greet you and the world. 

Your little one has a CRL of about 13 - 14 inches and has a strong grip.

Week 39 - 40

Normally this is when you will give birth and meet your baby. 

When your baby greets everyone it could: 

1. Weigh around 7 - 8 pounds. 
2. Be around 18 - 22 inches long with extended legs. 
3. Have lost all the slimy material and fine hair, it’s normal for the fine hair to disappear a few months after a baby is born. 

Now, you'll get to see your little one grow in front of you rather than on a screen. Welcome and enjoy parenthood! 

Click here to know what your symptoms will be every month.