How does my baby develop in the second trimester?

Dr. Deborah Raines, MSN
Nursing Specialist

At 20 weeks the fetus weighs about 10.5 ounces and is about 10 inches long: head to heel. The fetus is swallowing amniotic fluid, and producing meconium which will remain in the intestine until birth. The fetus is beginning to extend and flex its arm and legs and moving around in the uterus. Beginning at 20 weeks many of the fetus’ activities and behaviors are focused on refining body systems and functions to prepare for survival after birth.


At 25 weeks the fetus is about 9 inches in length and weight about 1.5 pounds. Capillaries are forming under the skin and the air sacs or alveoli in the lungs are developing. However these structures are not ready to function in the outside world yet. After this point, the baby begins developing and storing small amounts of fat in preparation for maintain body temperature in the outside world.

Things are really progressing with your baby by now! She is growing eyebrows and eyelashes as well as hair on her head. Her skin is still wrinkled and red, but she's slowly filling out. By the end of the second trimester, she will be about 14 inches long and weigh about 2 pounds. She'll move around freely—you'll know when she's wide awake and when she's resting. You may even feel her hiccup

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.

During the 2nd trimester of pregnancy, weeks 14 through 27, your baby will change and develop drastically. At week 14 your child is just beginning to develop facial features and genitalia, making it possible to determine your child's gender. By week 27 your child has advanced brain and lung function and could survive if delivered at this time. Because your child has also developed most of its senses at this time, they may be able to recognize your voice or feel your movements.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.