1. Pregnancy
  2. Pregnancy health
  3. Fetal development

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Fetal Brain Development Stages: When Does a Fetus Develop a Brain?

Your baby’s brain development is a complex process that continues throughout your pregnancy. At just six weeks, the embryo’s brain and nervous system begin to develop, although the complex parts of the brain continue to grow and develop through the end of pregnancy, with development ending around the age of 25. It’s important to take care throughout your pregnancy to ensure proper brain development for your baby.

Just a couple of weeks after you conceive, the embryo forms a neural plate. This is the base for the nervous system. As it grows, it becomes longer, folding in on itself to become the neural tube. The bulge of the tube becomes the brain, while the rest of the tube stretches into a spinal cord and eventually develops into the rest of the nervous system. 

The brain begins with the neural tube, formed in the first month of the embryo’s growth. The neural tube closes around week 6 or 7, at which point the brain separates into three parts: front brain, midbrain, and hindbrain. These three parts will eventually develop into the specialized parts of the brain, and the cerebrum will fold into the left and right halves of the brain.

From the time the neural tube closes, around week 7, the brain will grow at a rate of 250,000 neurons per minute for the next 21 weeks. Ultrasounds can reveal the embryo moving as early as 6 weeks after conception (or 8 pregnancy weeks), detecting the electrical impulses that govern movement and indicating that the brain is beginning to function.

During the first trimester, the brain develops rapidly and makes up nearly half of the fetus’s weight. For comparison, by the time your baby is born, the brain is only 10 percent of their body weight. In the first trimester, the brain will grow millions of neurons, which connect across synapses to direct movement and growth.

The communication between neurons is what helps the fetus learn to move, although during the first trimester you probably won’t feel any movement.

In the second trimester, the fetal brain begins to direct the compression of the chest muscles and movement of the diaphragm. These are kind of like practice breaths and are controlled by the brain stem. Sucking and swallowing begin around week 16, and by week 21, the fetus can swallow amniotic fluid.

During the second trimester, the fetus is still testing out movements, kicking and stretching. At some point between week 16 and week 20, the fetus should be large enough that you can feel the baby kick. These movements are directed by the cerebellum. At this point, the fetus develops the full range of specific fetal movements.

In the second trimester, the fetal brain begins to direct the compression of the chest muscles and movement of the diaphragm. These are kind of like practice breaths and are controlled by the brain stem.

The fetal brain stem is almost entirely developed by the end of the second trimester. This part of the brain is located just above the spinal cord. The nervous system has developed enough to detect loud noises from outside — you may feel the baby startle when there’s a loud clap of thunder or a car honking. The baby will start to identify the sound of your voice and may turn its head up toward the sound.

Finally, the brainwaves that occur during sleep, controlled by the hypothalamus, begin occurring around week 28. Your baby will begin experiencing sleep cycles, including REM sleep, the stage where dreaming occurs.

Brain development in the third trimester is marked by the rapid development of neurons in the brain and explosive growth. Your baby’s brain will triple in size during this time, growing from a little over 3 ounces to almost 11 ounces at birth.

The cerebrum will begin to develop grooves and ridges and separate into the left brain and right brain. The cerebellum is the fastest-growing part of the brain in the third trimester. This is the part responsible for motor control, so your baby will begin to move more, wiggling fingers and toes, stretching, and kicking. 

Brain development in the third trimester is marked by the rapid development of neurons in the brain and explosive growth. Your baby’s brain will triple in size during this time, growing from a little over 3 ounces to almost 11 ounces at birth.

The baby is large enough that you’ll notice quite a bit of movement, some of which may be a little uncomfortable. In addition, the fetal sensory system shows integration and functionality during this period.

Your baby’s brain will grow five main parts, each responsible for a different aspect of directing the body and (eventually) the mind and decision making:

  • The cerebrum is responsible for thinking, feeling, and memory. It’s the largest part of the brain and contains the cerebral cortex and the frontal and temporal lobes of the brain.
  • The cerebellum is responsible for directing the body’s motor control, from moving arms and legs to the fine motor skills of pinching and grasping.
  • The brain stem controls the vital functions that keep the body alive. These are mainly involuntary systems like heartbeat, blood pressure, and breathing. It also controls the digestive process, although hunger cues come from elsewhere.
  • The pituitary gland is in charge of releasing most of the hormones in the body that direct the metabolism, ovulation, growth, and more.
  • The hypothalamus regulates body temperature, hunger and thirst cues, sleep, and emotions.

Ultrasounds during your pregnancy will allow your doctor to monitor brain development and make sure each part is growing according to schedule. 

It’s important to consume at least 600 mg of folate, or folic acid, during the first trimester and 400 mg later in pregnancy. This vitamin supports brain and spinal development, ensuring that the spinal tube fuses correctly and stimulating the growth of the brain throughout your pregnancy. Prenatal vitamins, created specifically to deliver the nutrients necessary during pregnancy, contain extra folate. If you aren’t getting enough from your diet, consult with your doctor about the best kind of prenatal vitamins for you. You can also get folate naturally from your food, specifically flax seed, whole grains, and leafy green vegetables.

It’s important to consume at least 600 mg of folate, or folic acid, during the first trimester and 400 mg later in pregnancy. This vitamin supports brain and spinal development, ensuring that the spinal tube fuses correctly and stimulating the growth of the brain throughout your pregnancy.

Omega-3 fatty acids are another vital nutrient for brain growth. The brain is composed of mostly fat tissue, which makes these fatty acids especially important. Omega-3s are a structural fat in the brain, eyes, and nervous system, and they help develop the neural pathways and communication between the different parts of the brain. You can get these naturally though fatty fish like salmon or from walnuts, certain nut butters, and avocados. You can also take fish oil or another type of Omega-3 supplement. Be sure to speak with your obstetrician before taking supplements.

Eating a balanced diet of whole foods and drinking plenty of water will help your baby's brain develop properly. 

One of the largest contributors to fetal harm is alcohol. Drinking while pregnant can severely impact the growth and development of the baby’s brain. Alcohol consumption can cause fetal alcohol syndrome, which causes brain damage and problems with a baby’s growth. Babies with fetal alcohol syndrome may have a certain cast to their facial features, including drooping eyes. They may also experience speech delays and mild to severe retardation. There is no known “safe” amount of alcohol to consume while pregnant, and the effects of fetal alcohol syndrome are irreversible. If you are having trouble abstaining from drinking, it’s important to discuss this with your doctor right away.

One of the largest contributors to fetal harm is alcohol. Drinking while pregnant can severely impact the growth and development of the baby’s brain.

Smoking is also harmful to a baby’s development overall, including low birth weight and a reduction of the formation of neurons in the brain. Cigarette smoke, and the chemicals it contains, also impact the communication between neurons.

Illness and infections in the mother can have a profound impact on the development of the fetus’s brain. This can result in lowered neurological and psychomotor skills, like motor function, language, social adaptation, and cognitive function. Even relatively minor illnesses, such as chickenpox, can seriously damage the formation of brain cells and their wiring. Sexually transmitted infections are especially important to treat or monitor. 

If you don’t have a cat, it’s best to wait until after you have the baby to get one. Feline feces can contain parasites that cause toxoplasmosis, which can be harmful to your baby and especially their brain. If you do have a cat, get someone else to clean the litter box and be sure to wear gloves if you do it yourself. 

Ensuring that your baby has healthy brain development looks a lot like ensuring the overall health of your pregnancy. Eat whole foods and drink plenty of water. Be sure to take proper prenatal care, including supplements as directed by your doctor. Refrain from smoking and drinking, and be sure to get mild exercise as recommended by your OB-GYN. 

Borsani et al., Correlation between human nervous system development and acquisition of fetal skills:an overview. Brain and Development, vol41, issue 3, March 2019, 225-233.

Coletta et al., Omega-3 fatty acids and pregnancy. Review Obstetrics and Gynecology. 2010 Fall; 3(4):163-171

K.L Jones, The effect of alcohol on fetal development. Embryo today:Reviews. 21 March 2011

Wehby et al., The impact of maternal smoking during pregnancy on early child neurodevelopment. Journal of human capital, 20115(2): 207-254

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biointeractive.org/classroom-resourses/prenatal-development--human-brain

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