During the second trimester, the fetus’ growth rate accelerates significantly. The expectant mother will begin to appear more pregnant during the second trimester. She should continue the health regimen she established in the first trimester, with some exceptions. She should stop taking folic acid supplements and may have to modify her exercise regimen. In addition, women should avoid taking any type of medication unless a physician approves it.

The second trimester usually unfolds as follows:

• Week 13: The fetus begins to flex its arms and kick its legs, although the expectant mother may not be able to feel this. The fetus may even begin to suck its thumb. The eyelids become fused together to protect the eyes as they develop, and bone cells begin to develop around the baby’s head and within the arms and legs. The ribs begin to become visible on ultrasound.
  
  
• Week 14: The roof of the mouth completes formation. In boys, the prostate gland develops. In girls, the ovaries shift from the abdomen to the pelvis. The fetus’ intestinal tract creates meconium, which is the material of the baby’s first bowel movement following birth.
  
  
• Week 15: Nearly transparent skin forms, and eyebrows and scalp hair begin to emerge. The hair follicles begin producing pigment in children who will have dark hair. Bone and marrow continue to develop, and the eyes and ears continue to take on a more human appearance. The ears, which have gradually been moving toward the side of the head for several weeks, are nearly in their final destination.
  
  
• Week 16: The fetus’ eyes become sensitive to light, and facial muscles allow facial expressions such as squinting and frowning. The fetus can make a fist at this point, and may begin to hiccup frequently. For girls, the eggs form in the ovaries.
  
  
• Week 17: Fat stores begin to develop. They keep the baby warm after birth and provide energy.
  
  
• Week 18: The fetus may begin to hear at this point, detecting sounds such as the mother’s heartbeat, stomach rumbles or the sound of blood moving through the umbilical cord. The fetus also can swallow by this point.
  
  
• Week 19: A pasty, white coating called vernix protects the baby’s skin. Underneath the vernix, fine hair called lanugo covers the fetus’s body. The kidneys produce urine, which is excreted into the amniotic sac protecting the fetus. Hearing continues to develop. The baby can soon make reflexive muscle movements due to all of the motor neurons developing in the brain. The mother should be able to feel movement – also known as “quickening” – soon. Once quickening is felt, it should be reported to the physician. Knowing the starting time of quickening helps determine the baby’s due date.
  
  
• Week 20: The fetus’ skin continues to thicken and develop layers. Thin eyebrows and scalp hair are apparent on ultrasound. The arms and legs are well-developed. Week 20 marks the halfway point of the pregnancy. The fetus is about 6 inches long (15.24 centimeters) and weighs 9 ounces (255 grams). The fetus’ heartbeat can be heard with a stethoscope.
  
  
• Week 21: The fetus begins to develop new sources of nourishment aside from the placenta. For example, swallowing amniotic fluid provides small amounts of glucose (blood sugar). Bone marrow begins to make blood cells, which previously had been accomplished by the liver and spleen.
  
  
• Week 22: Taste buds begin to form, and nerve endings provide the fetus with the ability to process the feeling of touch. The fetus may begin to test this sensation by feeling around for things. In boys, testes begin to descend from the abdomen, while the uterus and ovaries in girls are already in place.
  
  
• Week 23: The lungs begin to produce surfactant, which allows air sacs (alveoli) in the lungs to inflate and keeps the sacs from sticking together when they deflate. The skin becomes less transparent and the fetus starts to add fat at an increased rate. By the 23^rd week, the fetus is potentially capable of surviving outside the womb. However, there are substantial health threats for babies born at this point, including bleeding in the brain, vision impairment and lung damage.
  
  
• Week 24: Footprints and fingerprints begin to form, and the fetus develops a sense of balance as the inner ear becomes fully developed.
  
  
• Week 25: The fetus’ hands are fully developed, and the fetus may begin to explore structures inside the uterus.
  
  
• Week 26: Eyebrows and eyelashes are well-formed, and the fetus has increasing amounts of hair. The eyes are fully developed but may not open for another couple of weeks.
  
  
• Week 27: The lungs, liver and immune system continue to evolve.