Fetal development: The second trimester

Fetal development takes on new meaning in the second trimester. Highlights might include finding out your baby's sex and feeling your baby move. By Mayo Clinic Staff

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.

Dec. 04, 2012 See more In-depth