How your baby’s brain develops during pregnancy


Can you imagine your baby’s brain is already developing even before you know you’re pregnant? Their brain grows at the supersonic rate of about 250,000 nerve cells per minute throughout pregnancy, setting the foundation for a lifetime’s worth of experiences.

Let’s look at how a journey that starts with a few neural cells creates the most complex organ known to human civilization.

First trimester

Your baby’s brain development journey starts in the fifth week of pregnancy. An embryo has three layers:

  • Ectoderm (the top layer)
  • Mesoderm (the middle layer)
  • Endoderm (the inner layer)

Each layer later develops into different parts of your baby’s body. During this time, the embryo’s ectoderm, or topmost layer, thickens and hardens into a neural plate. It forms the neural tube over the next few weeks, a structure that eventually becomes the brain and the spinal cord.


Did you know?

A baby is born with around 100 billion neurons, comparable to the number of stars in the Milky Way! These are all the neurons they will ever have in our lifetime.

By the end of the fifth week, the neural tube will start looking more like the brain. Your baby’s eyes begin forming around six weeks after conception.

After neurons begin to form, your baby’s brain becomes more organized as these neurons migrate to the correct part of the brain. From there, they form synapses which allow neural circuits to form.”

These neural circuits will continue to become more and more complex with time and dictate the skills and abilities your baby will acquire throughout their life.

The electrical activity begins in the brain as connections form, allowing your baby to coordinate their first spontaneous movements. However, in most cases, you can’t feel them yet. But the good news is that your doctor can detect the movements on an MRI or ultrasound. Fetuses can also respond to their mothers’ movements by kicking back.

Second trimester

As your second trimester begins, folds and grooves appear on your baby’s brain surface, giving it a wrinkled appearance (like that of an adult’s brain). These folds and grooves are landmarks that help us separate complex brain structures and their functions.

According to Dr. Radford, “As the brain develops, your baby can sense things like touch (and pressure), sound, heat, and light, and respond to these inputs by moving (or even becoming startled).”

Between 12 to 14 weeks gestation, the nerve cells in your baby’s brain are increasing to 15 million per hour! They also start migrating to different brain regions and simultaneously form more connections.


Did you know?

By the end of the first trimester, your infant’s brain looks similar to an adult’s, minus the folds.

The cerebellum (part of the brain responsible for motor control) is more developed and directs your baby’s movements. At around 18 weeks, your baby will start kicking. And this time, you’ll be able to feel it too. ️

During this trimester, your baby’s brain will direct their chest and diaphragm muscles to contract, which is like breathing practice for later.

By 13 weeks of pregnancy, your baby’s sucking and swallowing impulses will kick in. They will start swallowing the amniotic fluid at around 21 weeks. The area of their brain—the gustatory cortex, which is responsible for their sense of taste—is also developing. Through the amniotic fluid, your little one gets a taste and smell of the food you’re consuming. Another reason to sample as many foods as possible so that your baby gets maximum exposure to different flavors!

If a mother has a broader palate, her baby will be more willing to try new foods after they’re born. This is also true if [the] baby is breastfed; exposure to different flavors through mother’s milk makes a child more interested in trying a wider range of foods.”

Around this time, another process of ‘neuronal migration‘ also occurs in your baby’s brain.

Neuronal migration is basically neurons moving from when they are first developed into the nervous system area where they’re supposed to be,” says Dr. Radford, “If the neurons don’t go where they are supposed to go, connections will be missing, perhaps leading to dyslexia, epilepsy, or other issues.”

By the end of your second trimester, your baby’s brainstem (responsible for controlling heart rate, breathing, and blood pressure) is almost entirely developed. They can hear and recognize sounds outside the womb, including your voice. Dr. Radford states, “Babies have been seen to respond to touch between 21 to 25 weeks gestation.”

Your baby’s eyes will be a little more developed. They would be able to detect light coming in through the womb.

Third trimester

You’re now moving toward the end of your pregnancy and getting ready to welcome your baby into the world!

In the last three months of this journey, your baby’s brain will triple in size, forming neurons and neural connections at lightning speed.

The smooth surface of your little one’s brain will start developing deeper grooves and ridges, which will separate into the left brain and right brain. The left side of the brain is responsible for logic, calculations, mathematical understanding, logic, facts, and sequencing. On the other hand, the right side is in charge of visual awareness, imagination, interpreting social cues, and musical awareness, among many other things.


Did you know?

If our nerves were not covered with myelin, our spinal cord would have to be around nine feet wide to achieve the same efficiency in processing information.

Since the cerebrum is responsible for your baby’s motor control, its rapid development will result in more movement—your little one stretching, kicking, and wiggling their tiny fingers and toes.

Another exciting development during this time is that your baby starts sleeping.

Do babies sleep in the womb?

Well, a little bit of both. Up until 30 weeks, your baby spends most of their time sleeping. Then they begin to develop sleep-wake cycles. (They are falling asleep and waking up in organized patterns.) This development of a sleep-wake cycle can be attributed to the growing development of the baby’s nervous system.

Around the seventh month of pregnancy, your baby will exhibit their first rapid eye movements (or REM), indicating that they might be dreaming.


In conclusion, the journey of your baby’s brain development during pregnancy is nothing short of miraculous. From the initial formation of the neural tube to the intricate networks of neurons and synapses, every stage sets the stage for a lifetime of learning and experiences.

As your baby’s brain triples in size during the third trimester, the differentiation of the left and right hemispheres marks the beginning of cognitive specialization. Even before birth, your baby is responsive to stimuli from the external world, forming connections that will shape their understanding and interactions after birth.

Understanding the marvels of prenatal brain development not only deepens our appreciation for the complexity of life but also underscores the importance of nurturing environments for optimal cognitive growth.

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