When Does a Fetus Develop a Brain?

Watching your body change and your pregnant belly grow is an exciting time. But, if you think a lot is going on on the outside, that’s nothing compared to the changes that your little one is going through inside your body. Of all the processes they go through to get from fertilized egg to bouncing boy or girl, their brain and nervous system development is one of the longest and most complicated.
You’ve got a lot of time to think when you’re waiting for your baby. If you’ve found yourself asking “when does a fetus develop a brain?,” we’ve got the answers here.

In this article: 📝

When is a fetus considered a baby scientifically?

Let’s clear up some terminology.

From a cultural, personal, and emotional point of view, most parents-to-be will talk about the baby in the mama’s uterus. The words embryo and fetus are ones we read in pregnancy books. But, to discuss fetal development, your future baby is technically an embryo after the fertilized egg attaches to the uterine wall and starts growing into a separate placenta and baby. After the 10th week of pregnancy, doctors refer to the baby as a fetus until it’s born.

At what point does this fetus show brain activity?

The brain and nervous system is one of the very first things to start forming in your baby’s body. The processes get started before you even know you’re pregnant. But when do we see the first signs of brain activity in a growing fetus?

In week five of pregnancy, the brain cells start to form.

By week eight, the electrical activity (like the impulses that control the heartbeat) begins.

By week ten, the organ inside your baby’s head is a more recognizable brain.

Fetal brain development then continues for the rest of your pregnancy. And it’s a good job too, because it’s how your growing peanut starts to wriggle, stretch, yawn, and even suck their thumb in the womb.

When is a baby’s brain fully developed?

The science gets pretty complicated here but, in simple terms, the brainstem (which controls your baby’s heartbeat and breathing) continues to develop throughout the second trimester. After that, the cerebral cortex, which will be responsible for everything from processing information from their senses to storing their first memories, develops more and more quickly as the third trimester goes on.

Of course, it’s difficult to say when a baby’s brain is “fully” developed because they still have so much to learn after they’re born. In fact, brain development continues until we’re about 25 years old (or about 1,300 weeks, if you’re still counting).

Which trimester is most important for fetal brain development?

Although your baby’s brain needs your entire pregnancy to grow, there are still times where bigger leaps in fetal brain development are happening.

First trimester:

At around week six of pregnancy, the cells that’ll make the brain start to split off into the five different regions that we’re most familiar with: the cerebrum, cerebellum, brain stem, pituitary gland, and hypothalamus. The nervous system also starts forming, even if it’s going to be a few months before it can control those all-important first kicks.

Second trimester:

At the end of the second trimester, the brainstem is almost completely developed. At 28 weeks, doctors can monitor fetal brain activity and can even tell when your little one is dreaming.

Third trimester:

This is where the magic happens. Rapid fetal brain development in the final trimester gets your little one ready to experience the world, recognize you, and have (at least a little) control over their movements. The brain doubles in weight during these final three months as the cerebellum grows. Around your baby’s due date, the cerebral cortex starts to function.

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Embryo vs Fetus: What’s the Difference?