Fetal development: The 2nd 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. Two months ago, your baby was 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 is beginning to make urine and release it into the amniotic sac, making amniotic fluid. Yes, that's right. Your baby swallows the amniotic fluid and then urinates it back into the amniotic sac, making amniotic fluid.

Bones are beginning to harden in your baby's skeleton, especially in the head and long bones. Your baby's skin is still thin and transparent, but it will start to thicken soon.

Week 14: Baby's sex becoming apparent

Fourteen weeks into your pregnancy, or 12 weeks after conception, your baby's neck has become more defined and the lower limbs are well-developed. 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 might 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 scalp pattern develops

Fifteen weeks into your pregnancy, or 13 weeks after conception, your baby is growing rapidly. Bone development continues and will become visible on ultrasound images in a few weeks. Your baby's scalp hair pattern also is forming.

Week 16: Baby's eyes move

Sixteen weeks into your pregnancy, or 14 weeks after conception, your baby's head is erect. His or her eyes can slowly move. The ears are close to reaching their final position.

Your baby's limb movements are becoming coordinated and can be detected during ultrasound exams. However, these movements are still too slight to be felt by you.

By now your baby might be more than 4 1/2 inches (120 millimeters) long from crown to rump and weigh close to 4 ounces (110 grams).

Week 17: Baby's toenails develop

Seventeen weeks into your pregnancy, or 15 weeks after conception, toenails appear.

Your baby is becoming more active in the amniotic sac, rolling and flipping. His or her heart is pumping about 100 pints of blood each day.

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. The eyes are beginning to face forward. Your baby's digestive system has started working.

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 develops protective coating

Nineteen weeks into your pregnancy, or 17 weeks after conception, growth slows.

A greasy, cheeselike 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 vaginal canal are forming.

July 08, 2017 See more In-depth