Baby Development Facts

4 to 6 Weeks

By 5 weeks, development of the brain, spinal cord, and heart is well underway.

The heart begins beating at 5 weeks and one day and is visible by ultrasound almost immediately.

By 6 weeks, the heart is pumping the embryo’s own blood to his or her brain and body. All four chambers of the heart are present and more than 1 million heartbeats have occurred. The head, as well as the chest and abdominal cavities have formed and the beginnings of the arms and legs are easily seen.

6 to 8 Weeks

Rapid brain development continues with the appearance of the cerebral hemispheres at 6½ weeks.

The embryo reflexively turns away in response to light touch on the face at 7½ weeks.

Fingers are beginning to form on the hand.

8 to 10 Weeks

Brainwaves have been measured and recorded before 8½ weeks.

Also by 8½ weeks, the bones of the jaw and collar bone begin to harden.

By 9 weeks the hands move, the neck turns, and hiccups begin. Girls now have ovaries and boys have testes. The embryo’s heart rate peaks at about 170 beats per minute and will gradually slow down until birth.

Electrical recordings of the heart at 9½ weeks are very similar to the EKG tracing of a newborn. The heart is nearly fully formed.

By 10 weeks kidneys begin to produce and release urine, and intermittent breathing motions begin. All fingers and toes are free and fully formed, and several hundred muscles are present. The hands and feet move frequently and most embryos show the first signs of right- or left-handedness.

Experts estimate the 10-week embryo possesses approximately 90% of the 4,500 body parts found in adults. This means that approximately 4,000 permanent body parts are present just eight weeks after conception.

10 to 12 Weeks

After 10 weeks, the developing human is called a fetus, which means “little one” or “unborn offspring.”

The eyelids are temporarily fused together by 10½ weeks.

By 11 weeks the head moves forward and back, the jaw actively opens and closes, and the fetus periodically sighs and stretches. The face, palms of the hands, and soles of the feet are sensitive to light touch. Thumb sucking and swallowing amniotic fluid begin. Girls’ ovaries now contain reproductive cells which will give rise to eggs later in life. Also in girls, the uterus is now present.

Yawning begins at 11½ weeks.

The number of heartbeats now exceeds 10 million.

Fingerprints start forming at 12 weeks while fingernails and toenails begin to grow.

The bones are hardening in many locations.

12 to 14 Weeks

By 13 weeks the lips and nose are fully formed and the fetus can make complex facial expressions.

By 14 weeks taste buds are present all over the mouth and tongue.

The fetus now produces a wide variety of hormones.

Arms reach final proportion to body size.

14 to 16 Weeks

By 15 weeks the entire fetus (except for parts of the scalp) responds to light touch. Tooth development is underway.

Gender differences emerge at 16 weeks when girl fetuses move their jaws more often than boys.

A pregnant woman may begin to feel fetal movement at this time.

16 to 18 Weeks

Production of a variety of digestive enzymes is well underway.

Around 17 weeks blood cell formation moves to its permanent location inside the bone marrow and the fetus begins storing energy in the form of body fat.

By 18 weeks formation of the breathing passages, called the bronchial tree, is complete. The fetus releases stress hormones in response to being poked with a needle.

18 to 20 Weeks

By 19 weeks, more than 20 million heartbeats have occurred.

By 20 weeks the larynx or voice box begins moving in a way similar to the movement seen during crying after birth. The skin has developed sweat glands.

20 to 22 Weeks

At 21 weeks breathing patterns, body movements, and heart rate begin to follow daily cycles called circadian rhythms.

By 22 weeks the sense of hearing begins to function and the fetus starts responding to various sounds. All skin layers and structures are complete.

With specialized medical care some fetuses can survive outside the womb by 22 weeks with survival rates reported as high as 40% in some medical centers.

22 to 24 Weeks

Between 20 and 23 weeks rapid eye movements begin. These eye movements are similar to those seen when children and adults have dreams.

By 24 weeks more than 30 million heartbeats have occurred.

24 to 26 Weeks

By 25 weeks, breathing motions may occur up to 44 times per minute.

By 26 weeks sudden, loud noises may trigger a blink-startle response, which may increase movement, heart rate, and swallowing.

The lungs produce a substance necessary for breathing after birth.

26 to 28 Weeks

By 27 weeks the thigh bones and the foot bones are each about two inches long.

By 28 weeks the sense of smell is functioning and eyes produce tears.

28 to 30 Weeks

By 29 weeks, pupils of the eyes react to light.83

30 to 32 Weeks

By 31 weeks more than 40 million heartbeats have occurred. Wrinkles in the skin are disappearing as more and more fat deposits are formed.

By 32 weeks breathing movements occur up to 40 percent of the time.

32 to 34 Weeks

By 34 weeks true alveoli, or “air pocket” cells, begin developing in the lungs.

34 to 36 Weeks

The 36-week fetus weighs about 5¾ pounds and measures about 18½ inches from head to heel.

36 to 38 Weeks

By 37 weeks the fetus has a firm hand grip and the heart has beat more than 50 million times.

38 to 40 Weeks

At term, the umbilical cord is typically 20 to 24 inches long.

Labor is initiated by the fetus, ideally around 40 weeks, leading to childbirth.

At full-term birth, newborn babies typically weigh between 6 and 9 pounds and measure between 18 and 21 inches from head to heel.