During the second trimester, fetal development shifts from the formation of basic structures to rapid growth and functional maturation. Muscles strengthen and movements become more coordinated and frequent. Sensory systems develop rapidly, including hearing and touch, allowing the fetus to respond to sound and movement. The brain undergoes rapid growth and increasing specialization of regions. Many organ systems begin functioning in more integrated ways. By the end of the second trimester, the fetus approaches viability with medical support and shows increasingly complex physiological activity.

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Week 11: Complex Development

 

Bones begin to harden through ossification, especially in the skull and long bones. Tooth buds form beneath the gums and will later give rise to primary teeth. The intestines complete their movement into the abdominal cavity from the umbilical cord. Facial features continue refining, with the eyes moving closer together. The liver is very active and produces most of the fetus’s red blood cells. Fingers and toes move independently. The fetus can stretch and make small grasping motions.

 

Size: 4.1 cm (fig)

 

Week 12

 

Reflexes such as sucking, swallowing, and grasping are present. The kidneys begin producing urine, which contributes to amniotic fluid volume. External genitalia may be distinguishable. The digestive system begins wavelike movements. Facial muscles allow subtle expressions. The fetus practices breathing motions using amniotic fluid.

 

Size: 5.4 cm (lime)

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Week 13

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Rapid growth in length occurs as the body begins catching up with the head. Fingerprints form and remain unchanged throughout life. The fetus can move its arms and legs in a coordinated manner. Vocal cords begin developing in the larynx. Intestinal activity increases as swallowed fluid moves through the gut. The pancreas starts producing insulin.

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Size: 7.4 cm (peach)

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Week 14

 

The head becomes more upright as the neck lengthens. Facial proportions become more balanced. Coordinated limb movements increase, including flexing and extending joints. External genitalia are typically identifiable on ultrasound. Fine hair called lanugo begins to cover the body. The fetus practices sucking motions.

 

Size: 8.7 cm (lemon)

 

Week 15

 

Bones continue hardening, particularly in the limbs. Scalp hair pattern develops, though hair may not yet be visible. The fetus practices breathing movements more regularly. Taste buds develop and can respond to flavors in amniotic fluid. Sensory nerves continue maturing. Limb movements become smoother and stronger.

 

Size: ~10.1 cm (apple)

 

Week 16

 

Muscles strengthen and movements become more coordinated. The eyes move forward into a more typical facial position. The fetus can hold its head more upright. Blood vessels are visible beneath the thin skin. Facial expressions such as squinting may occur. The heart pumps approximately 25 quarts of blood per day.

 

Size: 11.6 cm (avocado)

 

Week 17

 

Fat tissue begins forming beneath the skin, contributing to temperature regulation. Sweat glands develop across the body. The umbilical cord thickens and strengthens. Hearing structures mature further, allowing sound transmission. The fetus responds to loud noises with movement. Skeletal muscle mass increases.

 

Size: 13 cm (pear)

 

Week 18

 

The fetus can hear sounds from both inside and outside the womb. Movements become stronger and more frequent. The nervous system undergoes rapid organization. Myelin begins forming around nerves, improving signal transmission. Sleep and wake cycles begin emerging.

 

Size: 14.2 cm (bell pepper)

 

Week 19

 

Vernix caseosa forms a protective coating over the skin. Sensory development accelerates, particularly touch. Brain regions specialize further. The fetus responds more consistently to sound and movement. The skin remains translucent and reddish. Limb movements are well coordinated.

 

Size: 15.3 cm (tomato)

 

Week 20

 

The mother typically feels fetal movement, known as quickening. The fetus develops a regular sleep-wake rhythm. Hair, eyebrows, and eyelashes are clearly present. The digestive system continues maturing. Meconium begins accumulating in the intestines. Organ systems function in a more integrated manner.

 

Size: 25 cm (banana)

 

Week 21

 

Bone marrow begins producing blood cells, gradually replacing the liver and spleen in this role. Movements become stronger and more coordinated as muscle tone improves. The digestive system matures further, with increased swallowing of amniotic fluid. The fetus practices complex limb movements. Eyelids and eyebrows are fully formed. The skin remains thin and wrinkled. Growth in length slows while weight gain increases.

 

Size: 26.7 cm (carrot)

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Week 22

 

The lungs begin producing small amounts of surfactant, a substance necessary for keeping air sacs open after birth. Skin appears wrinkled and reddish due to visible blood vessels. Brain growth accelerates rapidly. Sensory pathways connecting the brain and body strengthen. Early viability becomes possible with intensive medical care. The fetus responds to touch and sound.
 

Size: 27.8 cm (spaghetti squash)

 

Week 23

 

Blood vessels in the lungs continue developing. The fetus reacts to loud noises and sudden movements. Skin remains thin, fragile, and translucent. Survival outside the womb is still difficult but increasingly possible in specialized neonatal units. Brain wave activity becomes more complex. The fetus can hiccup and swallow regularly.
 

Size: 28.9 cm (large mango)

 

Week 24: Viability

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Lung air sacs (alveoli) develop further. The face is fully formed with distinct features. Brain activity becomes more organized and measurable. Survival rates improve significantly with modern medical support. The fetus practices breathing motions more frequently. The skin remains delicate but begins thickening. At this point, the fetus is commonly said to have reached the point of viability where it could survive outside the mother's womb with substantial medical intervention, though some fetuses may become viable slightly earlier or slightly later.
 

Size: 30 cm (ear of corn)

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