Mayo Clinic Health Library

Fetal development: The second trimester

Updated: 12-04-2012

As your pregnancy progresses, your baby might begin to seem more real. You might hear the heartbeat at your prenatal appointments, and your enlarging abdomen might 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. Keep in mind that measurements are approximate.

Week 13: Urine forms

Thirteen weeks into your pregnancy, or 11 weeks after conception, your baby's intestines have returned to his or her abdomen from the umbilical cord — where they've been growing for the past couple of weeks. 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.

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 final relative lengths 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. For girls, ovarian follicles begin forming. For boys, the prostate appears.

By now your baby might be almost 3 1/2 inches (90 millimeters) long from crown to rump and weigh about 1 1/2 ounces (40 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: Baby can make sucking motions

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. Your baby might be able to make sucking motions with his or her mouth.

Your baby's movements are becoming coordinated and can be detected during ultrasound exams.

By now your baby might be more than 4 1/2 inches (120 millimeters) long from crown to rump.

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. Your baby might begin to hear.

By now your baby might 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, a greasy, cheese-like coating called vernix caseosa begins to cover your baby. The vernix caseosa helps protect your baby's delicate skin from abrasions, chapping and hardening that can result from exposure to amniotic fluid.

For girls, the uterus and vagina might begin to form this week.

Week 20: The halfway point

Halfway into your pregnancy, or 18 weeks after conception, you might be able to feel your baby's first movements, also known as quickening. If you've been pregnant before, you might have begun feeling your baby's movements a few weeks ago.

By now your baby might be about 6 1/3 inches (160 millimeters) long from crown to rump.

Week 21: Baby can swallow

Twenty-one weeks into your pregnancy, or 19 weeks after conception, your baby is poised to gain more weight. By this week your baby is becoming more active and is able to 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 might be visible.

By now your baby might 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 descending from the abdomen. For girls, the uterus and ovaries are in place — complete with a lifetime supply of eggs.

With intensive medical care, some babies born this week might be able to survive.

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 growing on his or her head.

By now your baby might be about 8 inches (210 millimeters) long from crown to rump and weigh more than 1 1/3 pounds (630 grams).

Week 25: Baby responds to your voice

Twenty-five weeks into your pregnancy, or 23 weeks after conception, your baby's hands and startle reflex are developing. Your baby might be able to respond to familiar sounds, such as your voice, with movement.

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 might 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 and nervous system are continuing to mature — and he or she has likely been growing like a weed. Your baby's crown-to-rump length might have tripled since the 12-week mark.