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Fetal development: The second trimester
By Mayo Clinic staffHealthy pregnancy (16)
- Childbirth education: Get ready for labor and delivery
- Working during pregnancy: Do's and don'ts
- Pregnancy nutrition: Foods to avoid during pregnancy
- see all in Healthy pregnancy
First trimester (6)
- Fetal development: The first trimester
- First trimester pregnancy: What to expect
- Prenatal care: What to expect during the first trimester
- see all in First trimester
Second trimester (7)
- Fetal development: The second trimester
- Second trimester pregnancy: What to expect
- Prenatal care: What to expect during the second trimester
- see all in Second trimester
Third trimester (9)
- Fetal development: The third trimester
- Overdue pregnancy: What to do when baby's overdue
- Third trimester pregnancy: What to expect
- see all in Third trimester
Pregnancy problems (14)
- Heart problems and pregnancy: Know the risks
- High blood pressure and pregnancy: Healthy mom, healthy baby
- Epilepsy and pregnancy: What you need to know
- see all in Pregnancy problems
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Fetal development: The second trimester
Fetal development takes on new meaning in the second trimester. Highlights may include finding out your baby's sex and feeling your baby move.
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 you to put away your favorite jeans.
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. Find out what happens during the second trimester by checking out this weekly calendar of events.
Week 13: Urine forms
Thirteen weeks into your pregnancy, or 11 weeks after conception, your baby's intestines have moved from the umbilical cord to your baby's abdomen. Your baby is also beginning to form urine and discharge it into the amniotic fluid.
Tissue that will become bone is also developing around your baby's head and within his or her arms and legs. Tiny ribs may soon appear.
Week 14: Baby's sex becomes apparent
Fourteen weeks into your pregnancy, or 12 weeks after conception, your baby's arms have almost reached the length they'll be at birth and your baby's neck has become more defined. Red blood cells are forming in your baby's spleen.
Your baby's sex will become apparent this week or in the coming weeks.
By now your baby may be almost 3 1/2 inches (87 millimeters) long from crown to rump and weigh about 1 1/2 ounces (45 grams).
Week 15: Baby's skeleton develops bones
Fifteen weeks into your pregnancy, or 13 weeks after conception, your baby is growing rapidly. Your baby's skeleton is developing bones, which will become visible on ultrasound images in a few weeks. Your baby's scalp hair pattern also is forming.
Week 16: Facial expressions are possible
Sixteen weeks into your pregnancy, or 14 weeks after conception, your baby's eyes have begun to face forward and slowly move. The ears are close to reaching their final position. More-developed facial muscles may lead to various expressions, such as squinting and frowning. Your baby also can now make sucking motions with his or her mouth.
Although still too slight to be felt, your baby's movements are becoming coordinated and can be seen during ultrasound exams.
By now your baby may be more than 4 1/2 inches (120 millimeters) long from crown to rump and weigh nearly 4 ounces (110 grams).
Week 17: Fat accumulates
Seventeen weeks into your pregnancy, or 15 weeks after conception, fat stores begin to develop under your baby's skin. The fat will provide energy and help keep your baby warm after birth.
Week 18: Baby begins to hear
Eighteen weeks into your pregnancy, or 16 weeks after conception, your baby's ears begin to stand out on the sides of his or her head. 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.
By now your baby may be 5 1/2 inches (140 millimeters) long from crown to rump and weigh 7 ounces (200 grams).
Week 19: Baby's uterus forms
Nineteen weeks into your pregnancy, or 17 weeks after conception, 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.
For girls, the uterus and vagina may be forming this week.
Week 20: The halfway point
Halfway into your pregnancy, or 18 weeks after conception, your baby's delicate skin is protected with a greasy, cheese-like coating called vernix caseosa.
You may be able to feel your baby's first movements, also known as quickening.
By now your baby may be about 6 1/3 inches (160 millimeters) long from crown to rump and weigh more than 11 ounces (320 grams).
Week 21: Baby can swallow
Twenty-one weeks into your pregnancy, or 19 weeks after conception, your baby is about to gain more weight. By this week your baby is becoming more active and can swallow.
Week 22: Baby's hair becomes visible
Twenty-two weeks into your pregnancy, or 20 weeks after conception, your baby is completely covered with a fine, down-like hair called lanugo. The lanugo helps hold the vernix caseosa on the skin. Your baby's eyebrows may be visible.
By now your baby may be 7 1/2 inches (190 millimeters) long from crown to rump and weigh 1 pound (460 grams).
Week 23: Fingerprints and footprints form
Twenty-three weeks into your pregnancy, or 21 weeks after conception, your baby's skin is wrinkled, more translucent than before and pink to red in color.
This week your baby begins to have rapid eye movements. Your baby's tongue will soon develop taste buds. Fingerprints and footprints are forming. For boys, the testes are beginning to descend from the abdomen. For girls, the uterus and ovaries are in place — complete with a lifetime supply of eggs.
Week 24: Real hair grows
Twenty-four weeks into your pregnancy, or 22 weeks after conception, your baby is regularly sleeping and waking. Real hair is beginning to grow on his or her head.
By now your baby may be slightly longer than 8 inches (210 millimeters) from crown to rump and weigh more than 1 1/3 pounds (630 grams).
With intensive medical care, some babies born this week may be able to survive.
Week 25: Exploration continues
Twenty-five weeks into your pregnancy, or 23 weeks after conception, your baby's hands are fully developed — although the nerve connections to the hands have a long way to go. Exploring the structures inside your uterus may become baby's prime entertainment.
Week 26: Baby's fingernails develop
Twenty-six weeks into your pregnancy, or 24 weeks after conception, your baby has fingernails.
Your baby's lungs are beginning to produce surfactant, the substance that allows the air sacs in the lungs to inflate — and keeps them from collapsing and sticking together when they deflate.
By now your baby may be 9 inches (230 millimeters) long from crown to rump and weigh nearly 2 pounds (820 grams).
Week 27: Second trimester ends
This week marks the end of the second trimester. At 27 weeks, or 25 weeks after conception, your baby's lungs, liver and immune system are continuing to mature — and he or she has been growing like a weed. Your baby's crown-to-rump length may have tripled since the 12-week mark.
- Healthy pregnancy: Stages of pregnancy. U.S. Department of Health and Human Services. http://www.4women.gov/pregnancy/you-are-pregnant/stages-of-pregnancy.cfm#second. Accessed March 20, 2009.
- Moore KL, et al. The Developing Human: Clinically Oriented Embryology. 8th ed. Philadelphia, Pa.: Saunders Elsevier; 2003:4.
- Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. May 28, 2009.
