As pregnancy moves into the second trimester, your baby may begin to seem more real. At four weeks of pregnancy, your baby was a cluster of cells. At 11 weeks, the unborn baby became a fetus. Now, the organs, nerves, and muscles are starting to work. Use this information to follow along with what's happening week by week during the second trimester.

As you track your pregnancy, remember that to find your estimated due date, your healthcare professional counts ahead 40 weeks from the start of your last period. That means you're actually not pregnant the first week or two of what's counted as your pregnancy's 40-week timeline.

Thirteen weeks into pregnancy, or 11 weeks after conception, bones start to harden in your baby's skeleton, especially in the skull and long bones of the arms and legs. The skin is thin and transparent. But soon it will start to get thicker.

Fourteen weeks into pregnancy, or 12 weeks after conception, your baby's neck has become more defined. And red blood cells are forming in the spleen. Your baby's sex likely will become clearer this week.

By now, your baby might be almost 3 1/2 inches (87 millimeters) long from the top of the head to the bottom of the tailbone, called crown to rump, and weigh about 1 1/2 ounces (45 grams).

Fifteen weeks into your pregnancy, or 13 weeks after conception, your baby is growing quickly. Bone development continues. Soon you'll be able to see bones on ultrasound images. Your baby's scalp hair pattern also is forming.

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

Your baby is becoming more active, rolling and flipping. Although you might not be able to feel those movements yet, you may feel your baby hiccuping, which causes small jerking movements.

Eighteen weeks into pregnancy, or 16 weeks after conception, your baby's ears start to stand out on the head. Your baby also might begin to hear sounds. The digestive system has started working.

By now, your baby might be about 5 1/2 inches (140 millimeters) long from crown to rump and weigh 7 ounces (200 grams).

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

A greasy, cheeselike coating called vernix caseosa starts to cover your baby. The vernix caseosa helps protect your baby's delicate skin from scrapes, chapping, and hardening that can happen from being in contact with amniotic fluid.

Your baby also is starting to release urine that now makes up most of the amniotic fluid.

Halfway into your pregnancy, or 18 weeks after conception, you might be able to feel your baby's movements. This sometimes is called quickening. Your baby is regularly sleeping and waking and might be wakened by noises or your movements.

By now, your baby might be about 6 1/3 inches (160 millimeters) long from crown to rump and weigh about 11 ounces (320 grams).

Twenty-two weeks into pregnancy, or 20 weeks after conception, your baby's eyebrows and hair are visible.

In the male reproductive system, the testes have begun to move down into the scrotum. In the female reproductive system, the uterus and ovaries are in place, and the vagina is developed.

By now, your baby might be about 7 1/2 inches (190 millimeters) long from crown to rump and weigh about 1 pound (460 grams).

Twenty-three weeks into your pregnancy, or 21 weeks after conception, your baby starts to have rapid eye movements. Ridges also form in the palms of the hands and soles of the feet that will create the foundation for fingerprints and footprints.

The lungs are starting to make a substance called surfactant. It allows the air sacs in the lungs to inflate and keeps the lungs from collapsing and sticking together when they deflate.

If born preterm, some babies this far along in pregnancy may be able to survive outside the uterus.

Twenty-four weeks into pregnancy, or 22 weeks after conception, your baby's skin is wrinkled and see-through. The skin tends to have a pink or red tint due to blood moving through the blood vessels.

By now, your baby might be about 8 1/4 inches (210 millimeters) long from crown to rump and weigh more than 1 1/3 pounds (630 grams).

By 26 weeks into your pregnancy, or 24 weeks after conception, eyebrows and eyelashes have formed. Your baby's eyes also are developed, but they may not open for another two weeks.

By now, your baby might be 9 inches (230 millimeters) long from crown to rump and weigh nearly 2 pounds (820 grams).

This week marks the end of the second trimester. At 27 weeks, or 25 weeks after conception, your baby's nervous system is continuing to mature. Your baby also is gaining fat, which helps the skin look smoother. This fat gain will continue into the third trimester.

Thirteen weeks into your pregnancy, or 11 weeks after conception, your baby is beginning to make urine and release it into the surrounding amniotic fluid. Your baby also swallows some amniotic fluid.

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

Fourteen weeks into your pregnancy, or 12 weeks after conception, 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 might be almost 3 1/2 inches (87 millimeters) long from crown to rump and weigh about 1 1/2 ounces (45 grams).

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

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

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.

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 sounds. 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).

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.

Twenty-two weeks into your pregnancy, or 20 weeks after conception, your baby's eyebrows and hair are visible. Brown fat also is forming, the site of heat production.

For boys, the testes have begun to descend.

By now your baby might be 7 1/2 inches (190 millimeters) long from crown to rump and weigh about 1 pound (460 grams).

Twenty-four weeks into your pregnancy, or 22 weeks after conception, your baby's skin is wrinkled, translucent and pink to red because of visible blood in the capillaries.

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).

Twenty-six weeks into your pregnancy, or 24 weeks after conception, 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).

This week marks the end of the second trimester. At 27 weeks, or 25 weeks after conception, your baby's nervous system is continuing to mature. Your baby is also gaining fat, which will help his or her skin look smoother.