How Your Baby Grows in the First Trimester

Finding out you’re pregnant is a magical moment, and incredible things will happen right from the start of your first trimester. Your baby, which simply starts as a group of cells, will transform and grow into the amazing little human being you will soon be holding. Here’s all the ways your baby will grow in their first trimester of pregnancy.

Weeks 1-2

The first couple weeks of your pregnancy you might be surprised to learn your baby isn’t even growing – yet. Why? The start of your pregnancy journey begins at conception and that process happens over a couple of weeks.

Week 3

In week 3, your future baby turns from a fertilized egg into a zygote, marking the start of their development. What’s a zygote? It’s a single cell with chromosomes from both mom and dad.1 These chromosomes determine your baby’s gender and physical traits like the hair and eye color you’ll wonder about and gush over once their born. The zygote will divide from one cell into multiple cells, forming groups the size of a raspberry,2 which brings you to Week 4 of your pregnancy.

Week 4

The raspberry-sized cells start forming the embryo and the placenta.3 Over the next 9 months, the placenta has the important job of getting nutrients to the embryo so it grows into a strong and healthy baby. Check out our blog post on essential pregnancy nutrients to see which ones you need.

Week 5

Five weeks marks the second month of pregnancy. You’ll be full of excitement as you schedule your first prenatal visit around this time. Your baby, which is now the size of a sesame seed, looks a bit like a tadpole. It’s made of layers that will become organs and tissue over time.4 Already their brain and spinal cord are growing from the neural tube and the heartbeat you can’t wait to hear will start beating.4

Week 6

Your baby is growing, and this when the pregnancy hormones and symptoms are kicking in. Areas where your baby’s eyes, nose, mouth and ears will be start developing along with their tongue and vocal cords.5 Like a flower, little buds which will grow into tiny arms and legs begin to sprout from their body.5

Week 7

At week 7, your baby is the size of a blueberry. They still seem so small, but now is the time their nose, eyes, mouth and ears take their shape. Parts of the eyes that help them see, stomach, esophagus and digestive organs like their liver and pancreas start growing as well.6

Week 8

Having grown to the size of a kidney bean, your baby is growing fingers and toes on those stubs which will soon be arms and legs. Their fingers and toes are webbed like a duck now, but don’t worry, they won’t always be that way. Around this time nerves start to grow throughout and their respiratory system so they can breathe.7

Week 9

From a kidney bean to a grape, you’re now in your third month of pregnancy. Only 31 weeks and counting! At 9 weeks, they can flex elbows and knees,8 meaning they’re starting to move in your tummy even though you won’t be able to feel them just yet. Their mouth is also forming space where the teeth they’ll use to chew yummy food will eventually grow into.8

Week 10

At week 10, you’re almost to the end of your first trimester. Where did the time go? Your baby is now the size of a kumquat. Their eyelids have fully formed and will stay closed for the next 17 weeks, and the area where their teeth will be starts to harden.9

Week 11

Here’s to week 11! Your baby is the size of a fig, and your baby bump might start to proudly show a little. All of your baby’s vital organs should have finished developing at this point. You can also expect the organs that determine if your baby is a boy or girl, which you’ll soon celebrate with a gender reveal, will start taking shape.10

Week 12

The end of your first trimester is just around the corner. Your baby is the size of a lime, and it’s using its reflexes to move their fingers, toes, eyes and mouth. Their fingers and toes have lost their webbing, are becoming longer and are growing nails.11 In addition, your baby is also starting to grow soft peach fuzz all over their body.11

Week 13

Welcome to the last week of your first trimester – one trimester down, two more to go. Your baby is finally looking more and more like a little human being. At week 13, your baby is swallowing and using body fluids.12 Fingerprints as unique as they are, are starting to form on their tiny little fingers.12 Only the size of a peapod, it’s fascinating how they have grown so much since their first trimester and how they’ll continue growing through the rest of your pregnancy with many more weeks left to go.

Disclaimer: PerkinElmer does not endorse or make recommendations with respect to research, medication, or treatments. All information presented is for informational purposes only and is not intended as medical advice.


1. Cherry, Kendra. “The Importance of the Zygote Phase in Prenatal Development.” Verywell Family, 26 Jan. 2020,

2-3: Pevzner, Holly. “Week 3 of Your Pregnancy.” Verywell Family, 4 June 2020,

4-12: “First Trimester.” BabyCenter,

Leave a Reply

Your email address will not be published. Required fields are marked *