The following developments take place during the second trimester (Week 13-Week 28):
At 16 weeks:
• Muscle tissue and bone continue to form, creating a more complete skeleton.
• Skin begins to form. You can nearly see through it.
• Meconium (mih-KOH-nee-uhm) develops in your baby's intestinal tract. This will be your baby's first bowel movement.
• Your baby makes sucking motions with the mouth (sucking reflex).
• Your baby reaches a length of about 4 to 5 inches and weighs almost 3 ounces.
At 20 weeks:
• Your baby is more active. You might feel slight fluttering.
• Your baby is covered by fine, downy hair called lanugo (luh-NOO-goh) and a waxy coating called vernix. This protects the forming skin underneath.
• Eyebrows, eyelashes, fingernails, and toenails have formed. Your baby can even scratch itself.
• Your baby can hear and swallow.
• Now halfway through your pregnancy, your baby is about 6 inches long and weighs about 9 ounces.
At 24 weeks:
• Bone marrow begins to make blood cells.
• Taste buds form on your baby's tongue.
• Footprints and fingerprints have formed.
• Real hair begins to grow on your baby's head.
• The lungs are formed, but do not work.
• The hand and startle reflex develop.
• Your baby sleeps and wakes regularly.
• If your baby is a boy, his testicles begin to move from the abdomen into the scrotum. If your baby is a girl, her uterus and ovaries are in place, and a lifetime supply of eggs have formed in the ovaries.
• Your baby stores fat and has gained quite a bit of weight. Now at about 12 inches long, your baby weighs about 1½ pounds.
This answer is based on source ionformation from the National Women's Health Information Center.