How does a baby's brain develop in the womb, week by week?
The brain starts as the neural tube, which closes by about day 28 after conception. By week 6 to 7 (counted from your last period) it has formed five areas. Hearing begins around 19 weeks. In the third trimester the brain grows roughly four-fold and collects DHA faster than ever. Each stage relies on nutrients from you.
On this page
First, how weeks are counted
Doctors count pregnancy weeks from the first day of your last period, not from conception. So "week 5" is only about three weeks after conception. Everything below uses that standard count, except where a source counts days from conception.
The first trimester: building the frame
The first big event happens before many people know they're pregnant. A thin strip of cells along the embryo's back folds up and seals into a tube by about day 28 after conception, roughly six weeks after your last period. That tube becomes the brain and spinal cord. If it doesn't close fully, the result is a neural tube defect such as spina bifida. That's why the CDC recommends 400 mcg of folic acid a day starting at least a month before conception; see folic acid before pregnancy.
Then things move fast. From the US National Library of Medicine (MedlinePlus):
| Week (from last period) | What's happening |
|---|---|
| 5 | The brain, spinal cord and heart begin to develop. Nerve cells start forming. |
| 6-7 | The brain forms into 5 different areas. Some nerves running from the brain to the face and head are visible. Eyes and ears begin to form. |
| 8 | The brain keeps growing. Hands and feet look like little paddles. |
| 10 | The embryo is now called a fetus. Eyelids begin to close. |
| 11-13 | Your baby can make a fist. Nails appear. The face is well formed. |
Early in pregnancy your baby can't make its own thyroid hormone, which the brain and nervous system need to develop. The fetus depends entirely on yours, and your thyroid can't make it without iodine. In the first trimester, your iodine is your baby's iodine; see iodine in pregnancy. Folate, iodine, iron, vitamin D and choline are the supplies that matter most now.
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The second trimester: the baby starts listening
| Weeks | What's happening |
|---|---|
| 14-18 | Ears are still forming. |
| 16-24 | First movements, usually felt as flutters. |
| 18-21 | The anatomy scan usually happens in this window. Hearing begins to switch on. |
| 19 | The first responses to sound, starting with a low tone. |
| 19-27 | Responses to sound spread. By 27 weeks, 96% of babies respond to low tones. |
As hearing comes online, the brain begins filing away the sounds it hears most. The womb filters out high sounds and lets the melody and rhythm of speech through, and your voice comes through most strongly because it also travels through your body. Before birth, babies' heart rates react differently to their own mother's voice than to a stranger's. More in when babies can hear in the womb.
The third trimester: the biggest growth spurt
In the last trimester, your baby's brain grows at a pace that's hard to picture. Researchers at Children's National Hospital in Washington, DC, compared brain scans of healthy fetuses with those of babies born early, and described brain size growing roughly four-fold in the last trimester. Their headline message: each day a baby stays in the womb matters for normal brain development.
Several other building jobs peak now:
- DHA. The omega-3 fat builds up in the brain and eyes fastest in the third trimester. Researchers estimate a baby stores about 50 mg of DHA a day in this stretch.
- Myelin. The fatty coating that speeds nerve signals. A 2023 scan study found the brain's major wiring bundles gained myelin faster before birth than right after it.
- Iron stores. A full-term baby usually has enough iron saved to last about the first 4 to 6 months. Those stores are built from you.
- Memory for sounds. Babies who heard the same rhyme or melody often in late pregnancy reacted to it weeks later, even after birth.
A baby born at 37 weeks has had less of that growth time in the womb than one born at 40, even though both count as term. That's part of why late-pregnancy care focuses on reaching term safely.
What your baby's brain needs from you, by stage
| Stage | Key supplies | Easy moves |
|---|---|---|
| Before conception and weeks 1-12 | Folic acid, iodine | Prenatal with 400 mcg folic acid and 150 mcg iodine; iodized salt |
| Whole pregnancy | Choline (450 mg a day) | Two eggs most days, plus meat, fish, beans or milk |
| Second and third trimester | DHA | 8-12 oz of low-mercury fish a week, or ask about a supplement |
| Third trimester | Iron (27 mg a day target) | Iron-rich foods with vitamin C; ask for your ferritin |
Beyond food: go to sleep on your side from 28 weeks, learn your baby's movement pattern, and talk, read and sing to your bump. Calm, support and movement matter too.
After birth, the building speeds up
At birth, the brain already has most of the nerve cells it will ever have. What it doesn't have yet are the connections between them. The Harvard Center on the Developing Child puts the figure at more than 1 million new neural connections every second in the first few years of life, and by about age 6 a child's brain reaches roughly 90% of its adult size. Those connections come from ordinary moments: faces, voices, touch, play and sleep. The work you start in pregnancy keeps going for years.
Do this
- Take a prenatal with at least 400 mcg of folic acid and 150 mcg of iodine every day.
- Eat two eggs most days and 8 to 12 ounces of low-mercury fish a week.
- Ask for your iron results, including ferritin, and a recheck in the third trimester.
- Start a daily 10-minute talk, read or sing time with your bump by mid-pregnancy.
- Go to sleep on your side from 28 weeks and learn your baby's movement pattern.
When to ask your doctor
Check with your doctor or midwife first if…
- You've had a pregnancy affected by a neural tube defect, have diabetes or take seizure medicine (you may need a higher folic acid dose).
- You have a thyroid condition, before changing your iodine intake.
- You're thinking about adding any supplement beyond your prenatal.
Call your doctor now if…
- You have vaginal bleeding or fluid leaking.
- Your baby is moving less than usual or the pattern changes (call now, day or night, or 911).
- You haven't felt any movement by 24 weeks.
- You have a severe headache, vision changes or severe belly pain (call your doctor or 911 now).
This page is general information, not medical advice. Every pregnancy and every child is different. Ask your doctor, midwife or pediatrician before changing supplements, diet or care.
Grow a Genius
In the book: Chapter 14: Third Trimester: Getting Ready
Chapters 7, 10 and 14 follow your baby's brain through each trimester with what to eat, check and do at every stage, and The First 1,000 Days Calendar turns it into week-by-week pregnancy to-dos.
- The 7 Genius Builders, stage by stage
- 16 printable guides, including the Prenatal Label Decoder
- Built on 500+ sources from 35+ countries
- 90-day Double Guarantee
People also ask
When does a baby's brain start to form?
Very early. The neural tube, which becomes the brain and spinal cord, finishes forming by about day 28 after conception. By week 5 (counted from your last period) the brain, spinal cord and heart begin to develop, and by week 6 to 7 the brain has formed five areas.
When is the biggest period of brain growth in the womb?
The third trimester. Researchers describe brain size growing roughly four-fold in the last trimester, and the brain collects the omega-3 fat DHA fastest at this stage. That's why every week in the womb counts.
Can I do anything to help my baby's brain develop in the womb?
Yes, mostly simple things: folic acid before and in early pregnancy, enough iodine, choline from eggs and other foods, low-mercury fish for DHA, enough iron, and sleeping on your side late in pregnancy. Talking and singing to your bump starts the habits that matter after birth.
When can a baby hear in the womb?
Babies first respond to sound at about 19 weeks, starting with low tones. By 27 weeks, 96% respond to low tones, and higher notes follow by 33 to 35 weeks.