Brain Development – A Nine Month Miracle

By Dr Melodie de Jager |

Pregnancy is an unfolding miracle, where many things can go wrong—but 98% of the time, everything goes right.

FIRST TRIMESTER

Weeks 1–4

Brain development begins with the formation of the brain, nervous system, and spinal cord at the embryonic stage. From conception until the eighth week, the baby is known as an embryo. From the eighth week until birth, the baby is referred to as a fetus. By the end of the eighth week, the essential parts of the brain and central nervous system have formed, and the key components of the central and peripheral nervous systems are defined.

• Brain development is a lengthy process that plateaus around age 11 but continues throughout life.
• The “recipe” for brain development is written in the first few months of life.
• Medically, brain development begins in the third week after conception.
• From a neurodevelopmental perspective, it begins at conception, when a membrane envelops the fertilized egg. This membrane is crucial, as it produces three layers of stem cells from which: The skin, hair, nails, and teeth develop—to protect the baby. The digestive tract and metabolic functions form to nourish the baby. The brain, spinal cord, and nerves develop—to enable the baby to survive, laugh, walk, and talk later.

The brain is not just in the head—it runs to the tailbone.

Soon after implantation in the uterus, the neural tube forms. This tube must close properly. The top part becomes the skull-enclosed brain; the bottom forms the brain stem and spinal cord, stretching to the tailbone. The development of the backbone, heart, and blood circulation becomes a priority: the backbone organizes and protects nerve signals between the brain and body, while the cardiovascular system supplies oxygen—brain food—essential for brain development.

By week 4, the forebrain, midbrain, hindbrain, and optical stalk (the precursor to the eye) begin developing.

Baby is now the size of a poppy seed.

Weeks 5–11

During these six weeks, the baby’s organs begin to form, and some begin to function. The heart has already started beating—from one cell instinctively flexing, to clusters pulsing in unison, eventually forming four chambers.

The brain cannot operate in isolation—it needs a body with senses to send it information. The sense of touch—through the skin—is the first and largest sense to develop. Facial features begin to form to house the primary senses: movement and balance, smell, taste, hearing, and sight. Around week 5, buds appear that will grow into arms and legs, complete with fingers and toes.

With sufficient oxygen and glucose supply, the left and right brain hemispheres develop, ready to receive sensory input from—and send motor messages to—the opposite sides of the body.

The brain grows rapidly—becoming 10 times larger in just a few weeks—through genetic neurochemical processes and stimulation from the womb environment. No deliberate pre-birth stimulation is needed. The womb offers:

• Tactile input through constant massage.
• Vestibular stimulation through maternal movement and breathing.
• Smell and taste stimulation through amniotic fluid flavors.
• Hearing stimulation via the mother’s heartbeat, breath, digestion, voice, and external sounds.
• Sight receives the least input—eyes develop later and require darkness until other senses mature.

Because of rapid brain growth, the head is disproportionately large.

The hindbrain, responsible for heartbeat, breathing, and muscle movement, begins to regulate body functions. Around week 9, the Moro reflex appears, startling the embryo into muscle movement.

The Moro reflex initiates movement but only after the mouth, hands, and feet have developed. These parts then play a role in releasing stress hormones and grasping to self-soothe.

Neurons form at a rate of 250,000 per minute

The embryo is now a fetus. Vital organs—kidneys, intestines, liver—start to function, nourishing the brain.

As the fetus begins to resemble a human, it signals the start of a prolonged brain and nervous system development process. This continues for many years, with each body part maturing to fulfill its specialized role.

Baby is now the size of a fig.

Week 11

The spinal cord and nerves are clearly defined. Primitive reflexes—Moro, rooting, sucking, palmar, and plantar—initiate body movements and help the brain map the body. Rooting and sucking reflexes appear when the fetus grasps the umbilical cord or sucks its thumb—instinctive behaviors that help regulate stress.

There is still ample space in the womb, allowing the fetus to stretch, flex, and turn—though it’s too early for the mother to feel movement.

Week 13

The fetus is now fully formed. The next 28 weeks are devoted to developing the systems already in place. This marks the end of the first trimester.

Baby is now the size of a pea pod.

SECOND TRIMESTER

Week 14

The fetus receives nourishment through the placenta and practices breathing to supply the brain with nutrients. Primitive reflexes link senses to specific brain regions:

• Parietal lobes: skin and taste.
• Temporal lobes: balance, smell and hearing.
• Occipital lobes: sight.

These connections are made through the Moro, rooting, sucking, tonic labyrinthine, and asymmetrical tonic neck reflexes. It’s amazing that all of this occurs through instinctive movement—without conscious effort from the mother.

Week 18

The fetus becomes sensitive to light filtering through the womb, despite eyelids being closed. The brain rapidly forms millions of motor neurons, enabling voluntary muscle movements. The forebrain advances as complex neural connections form between senses, brain, and body.

Baby is now the size of a banana.

Week 22

The fetus begins distinguishing different sounds. Independent nerve cells start working together to form a functional nervous system.

Around week 25, the spinal cord begins to harden and straighten. The nervous system becomes more capable of controlling breathing, and the optic nerve grows highly responsive to light.

By week 26, the once smooth brain surface begins to develop creases. Stronger brain waves enhance hearing and vision.

The fetus practices breathing by inhaling and exhaling amniotic fluid. If born at 27 weeks, the baby’s lungs could function with medical support. The eyes can now open and close.

Baby is now the size of a pineapple.

THIRD TRIMESTER

The brain now regulates breathing and body temperature and continues developing cortical folds.

From week 29, the brain, lungs, and eyes grow rapidly.

Between weeks 34–37, the baby is considered “late preterm.” This is a vulnerable stage, as brain development accelerates dramatically.

Brain Development 20 – 40 weeks (Chan & Armstrong)

 Brain development at 20 weeks, 35 weeks and 40 weeks.

Babies born between 34 and 37 weeks need special care to support ongoing brain development. By 38 weeks, the baby is full-term, with mature lungs—but it’s ideal for the baby to remain in utero for optimal brain development.

If early delivery is necessary for medical reasons, developmental support will help ease the transition from womb to world.

At birth, the human brain is far more immature than that of other animals. It takes another 18 months for a baby to reach the level of independence seen in newborn animals. The nine months in the womb and the first 18 months of life are crucial for wiring a healthy brain.

The brain increases in size by four-fold during the preschool period, reaching approximately 90% of adult volume by age 6 — Allan Reiss

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