Week 13 The baby’s eyes and ears are moving closer to their final positions. The liver is secreting bile; the pancreas is producing insulin; the vocal cords are forming; and what will be later known as “baby teeth” are already positioned in the gums.
Week 14 Your baby is at least three and a half inches long. Lung power gains momentum as your baby “breathes” amniotic fluid in and out. Week 15 The body is now more proportionate to the head. Your baby has started thumbsucking. The skin is covered with lanugo—soft, velvety hair that protects the fetus until birth. Week 16 Expect some painless contractions every now and then for two reasons: one, your baby begins to “hiccup” quietly, and two, your baby’s bones are hardening, causing his fully functional joints to jerk. Facial muscles are developing. Week 17 Your baby is roughly the size of an avocado, and has pink, transparent skin. The major organs are in full swing. The heart is pumping approximately 25 quarts of blood a day. Week 18 The ears have reached their final position, and the eyes are now looking forward. You will feel swift jerks as your little one becomes more active. The baby’s skeleton is still soft and pliable, while the brain is growing fast.
Week 19 Skin is thicker, whitish, and covered with a creamy translucent moisturizer called vernix caseosa. Brown fat deposits begin to accumulate underneath the baby’s skin to help stabilize body temperature. Week 20 If you have a girl, her uterus is now developing. Your baby is now taking on a typical newborn’s sleeping patterns. Week 21 Your baby’s bone marrow starts helping the liver and spleen produce blood cells. Week 22 Your baby’s senses of taste and touch have started to develop. If you have a boy, the testes are now descending from the pelvis to the scrotum. If you have a girl, the vagina has developed and the uterus and ovaries are in position. Week 23 The lungs are starting to develop.
Week 24 The lungs are preparing for the initial secretion of surfactant, a fatty substance that will coat the inner lining of the air sacs. Surfactant keeps air sacs in the lungs from sticking together when your baby exhales outside the womb for the first time. Week 25 Your baby is growing more rapidly. The spine and the blood vessels of the lungs are beginning to form. The nostrils have started to open, and the bones are continuing to harden (ossification). Week 26 The air sacs are finally forming this week, and the lungs are already beginning to secrete surfactant. You can also see your baby’s veins through his or her skin. Week 27 You might feel more “filled up” since your baby is around 14.4 inches long (with legs extended). Neural connections are firing up. The lungs, liver, and immune system are still undergoing further development.