Pregnancy
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How your baby’s brain develops during pregnancy
Medically Reviewed Kecia Gaither, MD, MPH, MS, MBA, FACOG, FAIUM, double certified in OB/GYN and Maternal Fetal Medicine and an Associate Professor of Clinical Obstetrics and Gynecology at Weill Cornell Medicine
Written by Cradlewise Staff
Can you imagine your baby’s brain is already developing even before you know you’re pregnant? Throughout pregnancy, their brain forms new neurons at an average of roughly 250,000 every minute, though research shows that pace is far from constant. Most neurons form in the first four months, so during that window, the real rate can climb past 500,000 a minute.
Every week brings new structures, new connections, and new capabilities, even though almost none of it is visible from the outside. Your prenatal visits and ultrasounds offer small windows into this process, but there’s a lot happening between appointments that’s worth understanding too.
Let’s look at how a journey that starts with a few neural cells creates the most complex organ known to human civilization.
Furthermore, we spoke to Kecia Gaither, MD, MPH, MS, MBA, FACOG, FAIUM, double board-certified in OB/GYN and Maternal Fetal Medicine and an Associate Professor of Clinical Obstetrics and Gynecology at Weill Cornell Medicine, and Dr. Samantha Radford (PhD), an exposure scientist with a focus on maternal-child health, and the owner of Evidence-Based Mommy to learn what maternal-fetal medicine specialists watch for along the way.
In which week baby brain development during pregnancy?
Your baby’s brain starts forming in week 5 of pregnancy, when the neural tube develops. Brain cells are produced fastest from about week 12 to 14, the surface starts folding between weeks 10 and 15, and the brain grows most in the third trimester.
| Weeks | Brain development |
| 5 | Neural tube forms (becomes the brain and spinal cord) |
| From about 10 | First spontaneous movements, driven by early brain activity |
| About 12 to 14 | Brain cells (neurons) produced at their fastest |
| 16 | Connections (synapses) between neurons start forming |
| 18 to 22 | Detailed anatomy scan checks brain structures |
| Between 10 and 15 | Brain surface starts folding into grooves and ridges |
| 21 to 25 | Baby responds to touch |
| 25 | First response to sound |
| Around 28 | First rapid eye movements (REM-like sleep) |
| 28 to 40 | Rapid brain growth and deeper folding |
Note: Weeks of pregnancy, counted from your last period
First trimester
Neural tube formation
Your baby’s brain development journey starts in the fifth week of pregnancy, as per NHS UK. 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?
At birth, your baby’s brain contains around 100 billion neurons, comparable to the number of stars in the Milky Way. These are all the neurons they will ever have in their 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 22 days of their fetal life, as per the American Journal of Neuroradiology.
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.
First neurons and early movement
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, but your doctor can detect the movements on an MRI or ultrasound. Fetuses can also respond to their mothers’ movements by kicking back.
This early period is crucial. Learn why.
Some major anomalies of the head, brain, and spine can be picked up this early with specialized first-trimester ultrasound in certain situations, but because the brain is still developing so rapidly, many finer structures simply aren’t visible yet.
Diabetes and early brain development
It’s a common question: does gestational diabetes affect fetal brain development?
“ Pregestational diabetes has the clearest association with congenital malformation involving the fetal heart, CNS, and limbs, since these structures form within the first trimester. Glucose control in the preconception period is therefore exceedingly important, as abnormal glycemic status is teratogenic and can impact normal development of the heart, brain, spinal cord, and limbs.”
The heart, brain, and limbs all take shape in the first trimester, often before someone realizes they’re pregnant, which is why glucose control while trying to conceive, not just during pregnancy, makes a huge difference.
Gestational diabetes, which is usually diagnosed later, tends to develop after these structures have already formed, so it carries a different risk profile.
Second trimester
Sensory development
As your second trimester begins, folds and grooves appear on your baby’s brain surface, giving it a wrinkled appearance, similar to that of an adult’s brain. These folds and grooves are landmarks that help us separate complex brain structures and their functions.
By 13 weeks of pregnancy, your baby’s sucking and swallowing impulses will kick in. They will start swallowing amniotic fluid around 21 weeks. The gustatory cortex; the area responsible for their sense of taste is also developing, so your baby gets a taste and smell of the food you’re eating through the amniotic fluid.
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 too, and would be able to detect light coming in through the womb. Around 18 weeks, your baby will start kicking and this time, you’ll be able to feel it.
Around this time, another process of ‘neuronal migration‘ also occurs in your baby’s brain.
Neuronal migration
Between 12 to 14 weeks gestation, your baby’s brain is adding new nerve cells at a rate of 15 million per hour, as per the University of Queensland. They also start migrating to different brain regions and simultaneously form more connections.
Frontal lobe and hemisphere development
The mid-trimester, usually at weeks 18 to 22, is when your doctor takes the closest look at your baby’s brain structure. This is the detailed anatomy scan, and it’s the main window your care team uses to assess brain-related anomalies.
“ Prenatal ultrasound assesses whether the fetal brain is developing with the expected size, symmetry, anatomy, and fluid spaces for the gestational age. The standard views evaluate key structures, which include the lateral ventricles, CSP, midline falx, thalami, cerebellum, and cisterna magna. The gyri (grooves) of the brain are also visualized.”
In simpler terms: Your doctor is checking that your baby’s brain is the right size, both sides look symmetrical, the fluid-filled spaces look normal, and the surface is starting to develop its characteristic folds.
The frontal lobe, the region behind your baby’s forehead responsible for reasoning, planning, and impulse control, begins taking shape now too, though it’s one of the slowest parts of the brain to mature. It won’t be fully developed until well into your child’s twenties.
If something like ventriculomegaly, a mild widening of the fluid-filled spaces in the brain, shows up on a scan, your doctor may recommend closer monitoring.
As per Dr. Kecia Gaither: Ventriculomegaly is one example of a finding that requires closer, regular review, as it can impact fetal head size and brain mass, with outcomes depending on the underlying cause.
A finding like this is diagnosed further and monitored closely.
Third trimester
You’re now moving toward the end of your pregnancy and getting ready to welcome your baby into the world.
Brain growth and hemisphere specialization
In the last three months of this journey, your baby’s brain volume roughly doubles and cortical surface area roughly triples, 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, known as left and right hemispheres. 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 weren’t covered with myelin, our spinal cord would need to be 1 meter wide to process information at the same speed.
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.
A normal mid-trimester scan is reassuring, but it can’t rule out every condition.
According to Dr. Gaither, some conditions are progressive or only become visible later in pregnancy, including bleeding, infection-related brain injury, evolving ventriculomegaly, or differences in how the brain’s surface folds. That’s part of why later scans and checkups still matter, even after a clear anatomy scan.
Sleep-wake cycles and REM
Another exciting development during this time is that your baby starts sleeping.
Up until around 30 weeks, your baby spends most of their time sleeping. Then they begin to develop sleep-wake cycles, falling asleep and waking up in organized patterns, a development that can be attributed to the growing maturity of the baby’s nervous system.
Around the seventh month of pregnancy or by about week 28, your baby will exhibit their first rapid eye movements or REM, indicating they might be dreaming.
If you want to learn about babies dreaming in the womb, our post on Do babies sleep in the womb might interest you.
Did you know?
When a baby is born, their brain is only a quarter of the size of an adult’s brain and by age 5, their brain is 90% of its full size.
How nutrition supports your baby’s brain during pregnancy
What you eat plays a real role in how your baby’s brain develops, particularly nutrients like folate, choline, iron, and omega-3s.
Our full guide on foods for your baby’s brain development breaks down exactly what to prioritize and when.
How to improve baby brain development during pregnancy
While some of this is outside anyone’s control, a few things can help.
Apart from folic acid, Dr. Gaither points to a short, practical list: Apart from taking prenatal vitamins, avoid alcohol and recreational drugs, stay current on recommended vaccinations, and reduce exposure to infection risks like handling cat litter, which carries a risk of toxoplasmosis.
Cradlewise tip: If you are trying to conceive, starting folic acid now is worth considering. Neural tube formation happens before you even know you are pregnant, so waiting until you get a positive pregnancy test can mean missing that crucial window.
We have a post on how to stimulate your baby’s brain development in the womb that can be helpful.
Conclusion
The journey of your baby’s brain development during pregnancy is nothing short of remarkable. From the initial formation of the neural tube to the intricate networks of neurons and synapses, every stage sets up a lifetime of learning and experiences.
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.
FAQs
Q: When does a baby's brain start developing in the womb?
A: Around week 5 of pregnancy, when the embryo’s ectoderm layer begins forming the neural tube, the structure that becomes the brain and spinal cord.
Q: When does a baby's brain fully develop?
A: Not at birth. While the core structures form during pregnancy, the frontal lobe, responsible for reasoning and impulse control, continues maturing well into a person’s twenties.
Q: Do babies dream before birth?
A: Possibly. Around the seventh month of pregnancy, babies show their first rapid eye movements (REM), a pattern associated with dreaming, though no one can say for certain what, if anything, they’re dreaming about.
Q: What is the frontal lobe, and when does it develop?
A: The frontal lobe sits behind the forehead and handles reasoning, planning, and impulse control. It begins forming around the second trimester but is one of the last brain regions to fully mature; it continues to mature well into a person’s twenties.
Q: What does a doctor actually look for on an ultrasound to check brain development?
A: Size, symmetry, and the fluid-filled spaces in the brain, along with key structures like the lateral ventricles, thalami, and cerebellum. This is done as part of the detailed anatomy scan in your mid-trimester, around 18 to 22 weeks.
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- What to do when labor starts: A stage-by-stage guide.
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Sources:
- Rate at which neurons form during pregnancy. NIH. 1992. The Development and Shaping of the Brain.
- Your baby’s brain development journey starts in the fifth week of pregnancy. NHS UK. Week 5.
- Baby’s brain at birth contains around 100 billion neurons. Time. 1997. Fertile Minds.
- Baby’s eyes begin forming around 22 days of their fetal life. American Journal of Neuroradiology. 2009. In Utero Eye Development Documented by Fetal MR Imaging.
- Brain surface starts folding into grooves and ridges. Brain Cogn. 2011. The Development of Gyrification in Childhood and Adolescence.
- Baby’s response to outside sounds. Charlotte Lozier Institute. Hearing in the womb.
- Baby’s brain is adding new nerve cells at a rate of 15 million per hour. University of Queensland. Brain development by numbers.
- Our spinal cord would need to be 1 meter wide. Current Biology. 2016. Myelination.
- Baby’s brain at birth is only a quarter of the size of an adult’s brain. University of Queensland. Brain development by numbers.
- First rapid eye movements (REM-like sleep). Charlotte Lozier Institute. Month 7.
- Connections (synapses) between neurons start forming. Trends Cogn Sci. 2017. Functional connectivity of the human brain in utero.
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