Healthy pregnancy (16)
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First trimester (5)
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Second trimester (7)
- Second trimester pregnancy: What to expect
- Prenatal care: What to expect during the second trimester
- Fetal development: What happens during the second trimester?
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Third trimester (9)
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Fetal development: What happens during the second trimester?
Fetal development takes on new meaning in the second trimester. Your baby will begin to look like a newborn — and may even be able to hear you!
By Mayo Clinic staffAs your pregnancy progresses, your baby may begin to seem more real. You may hear the heartbeat at your prenatal appointments, and your enlarging abdomen may force your favorite jeans to the back of the closet.
While you're adjusting to the changes in your body, fetal development takes on new meaning. Two months ago, your baby was simply a cluster of cells. Now he or she has functioning organs, nerves and muscles. You may be amazed by how much your baby changes from week to week.
Week 13: Baby flexes and kicks
You can't feel it yet, but your baby can move in a jerky fashion — flexing the arms and kicking the legs. This week, your baby might even be able to put a thumb in his or her mouth.
Your baby's eyelids are fused together to protect his or her developing eyes. Tissue that will become bone is developing around your baby's head and within the arms and legs. Tiny ribs may soon appear.
Week 14: Hormones gear up
The effect of hormones becomes apparent this week. For boys, the prostate gland is developing. For girls, the ovaries move from the abdomen into the pelvis.
Meconium — which will become your baby's first bowel movement after birth — is made in your baby's intestinal tract. By the end of the week, the roof of your baby's mouth will be completely formed.
Week 15: Skin begins to form
Your baby's skin starts out nearly transparent. Eyebrows and scalp hair may make an appearance. For babies destined to have dark hair, the hair follicles will begin producing pigment.
The bone and marrow that make up your baby's skeletal system are continuing to develop this week. Your baby's eyes and ears now have a baby-like appearance, and the ears have almost reached their final position.
Week 16: Facial expressions are possible
Sixteen weeks into your pregnancy, your baby is between 4 and 5 inches long and weighs a bit less than 3 ounces. He or she can now make a fist.
Your baby's eyes are becoming sensitive to light. More developed facial muscles may lead to various expressions, such as squinting and frowning. Your baby may have frequent bouts of hiccups as well. For girls, millions of eggs are forming in the ovaries.
Week 17: Fat accumulates
Fat stores begin to develop under your baby's skin this week. The fat will provide energy and help keep your baby warm after birth.
Week 18: Baby begins to hear
As the nerve endings from your baby's brain "hook up" to the ears, your baby may hear your heart beating, your stomach rumbling or blood moving through the umbilical cord. He or she may even be startled by loud noises. Your baby can swallow this week, too.
Week 19: Lanugo covers baby's skin
Your baby's delicate skin is now protected with a pasty white coating called vernix. Under the vernix, a fine, down-like hair called lanugo covers your baby's body.
Your baby's kidneys are already producing urine. The urine is excreted into the amniotic sac, which surrounds and protects your baby.
As your baby's hearing continues to improve, he or she may pick up your voice in conversations — although it's probably hard to hear clearly through the amniotic fluid and protective paste covering your baby's ears.
Thanks to the millions of motor neurons developing in the brain, your baby can make reflexive muscle movements. If you haven't felt movement yet, you will soon.
Week 20: The halfway point
Halfway into your pregnancy, your baby is about 6 inches long and weighs about 9 ounces — a little over half a pound. You've probably begun to feel your baby's movements.
Under the protection of the vernix, your baby's skin is thickening and developing layers. Your baby now has thin eyebrows, hair on the scalp and well-developed limbs.
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