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Fetal development: The third trimester

By Mayo Clinic staff
Original Article:  http://www.mayoclinic.com/health/fetal-development/PR00114

Fetal development: The third trimester

Fetal development continues during the third trimester. Your baby will open his or her eyes, gain more weight, and prepare for delivery.

By Mayo Clinic staff

The end of your pregnancy is near! By now, you might be tired of being pregnant — and eager to meet your baby face to face. Your uterus, however, is still a busy place. Understand how fetal development continues as you approach your due date. Here's a weekly calendar of events for the third trimester. Keep in mind that measurements are approximate.

Week 28: Baby's eyes open

Twenty-eight weeks into your pregnancy, or 26 weeks after conception, your baby's eyelids are partially open and eyelashes have formed. Your baby is gaining weight, which is smoothing out many of the wrinkles in his or her skin.

By now your baby might be nearly 10 inches (250 millimeters) long from crown to rump and weigh nearly 2 1/4 pounds (1,000 grams). Otherwise healthy babies born this week have a 90 percent chance of survival without physical or neurological impairment — and the odds improve with each passing week.

Week 29: Baby's bones are fully developed

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Illustration of fetus 27 weeks after conception Fetal development 27 weeks after conception

Twenty-nine weeks into your pregnancy, or 27 weeks after conception, your baby's bones are fully developed, but they're still soft and pliable.

Week 30: Baby's eyes are wide open

Thirty weeks into your pregnancy, or 28 weeks after conception, your baby's eyes are wide open a good part of the time. Your baby might have a good head of hair by this week. Red blood cells are now forming in your baby's bone marrow.

By now your baby might be more than 10 1/2 inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds (1,300 grams).

Week 31: Sexual development continues

Thirty-one weeks into your pregnancy, or 29 weeks after conception, your baby's central nervous system has matured to the stage where it can control body temperature.

Week 32: Baby practices breathing

Thirty-two weeks into your pregnancy, or 30 weeks after conception, your baby's toenails are visible.

Although your baby's lungs aren't fully formed, he or she practices breathing. Your baby's body begins absorbing vital minerals, such as iron and calcium from the intestinal tract. The layer of soft, downy hair that has covered your baby's skin for the past few months — known as lanugo — starts to fall off this week.

By now your baby might be 11 inches (280 millimeters) long from crown to rump and weigh 3 3/4 pounds (1,700 grams).

Week 33: Baby detects light

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Illustration of fetus 31 weeks after conception Fetal development 31 weeks after conception

Thirty-three weeks into your pregnancy, or 31 weeks after conception, your baby's pupils can constrict, dilate and detect light entering his or her eyes.

Week 34: Baby's fingernails grow

Thirty-four weeks into your pregnancy, or 32 weeks after conception, your baby's fingernails have reached his or her fingertips.

By now your baby might be nearly 12 inches (300 millimeters) long from crown to rump. The pasty white coating that protects your baby's skin — the vernix caseosa — is about to get thicker.

Week 35: Protective coating thickens

Thirty-five weeks into your pregnancy, or 33 weeks after conception, your baby's limbs are becoming chubby. Your baby is gaining weight rapidly — about 1/2 pound (230 grams) a week for the next month.

Week 36: Rapid weight gain begins

Thirty-six weeks into your pregnancy, or 34 weeks after conception, the crowded conditions inside your uterus might make it harder for your baby to give you a punch. However, you'll probably still feel lots of stretches, rolls and wiggles. You might want to check on your baby's movements from time to time (kick count) — especially if you think you've noticed decreased activity. Ask your health care provider how many movements you should detect in a certain number of hours.

Week 37: Baby is full term

Thirty-seven weeks into your pregnancy, or 35 weeks after conception, your baby will be considered full term. Your baby's organs are ready to function on their own.

To prepare for birth, your baby's head might start descending into your pelvis. If your baby isn't head down, your health care provider will talk to you about ways to deal with this issue.

Week 38: Baby develops a firm grasp

Thirty-eight weeks into your pregnancy, or 36 weeks after conception, your baby is developing a firm grasp.

Your baby's toenails have reached the tips of his or her toes. His or her brain might weigh about 14 ounces (400 grams). After birth, your baby's brain will continue to grow. Your baby has mostly shed all of his or her lanugo.

By now your baby might weigh about 6 1/2 pounds (2,900 grams).

Week 39: Placenta provides antibodies

Thirty-nine weeks into your pregnancy, or 37 weeks after conception, your baby's chest is becoming more prominent. For boys, the testes continue to descend into the scrotum.

The placenta continues to supply your baby with antibodies that will help fight infection after birth. If you breast-feed your baby, your milk will provide additional antibodies.

Week 40: Your due date arrives

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Illustration of fetus 38 weeks after conception Fetal development 38 weeks after conception

Forty weeks into your pregnancy, or 38 weeks after conception, your baby might be about 18 to 20 inches (450 to 500 millimeters) long and weigh 6 1/2 pounds (2,900 grams) or more. Remember, however, that healthy babies come in different sizes.

Don't be alarmed if your due date comes and goes without incident. It's just as normal to deliver a baby a week or two late — or early — as it is to deliver on your due date.

References
  1. Healthy pregnancy: Stages of pregnancy. U.S. Department of Health and Human Services. http://www.womenshealth.gov/pregnancy/you-are-pregnant/stages-of-pregnancy.cfm. Accessed Nov. 16, 2012.
  2. Frequently asked questions. Pregnancy FAQ156. How your baby grows during pregnancy. The American Congress of Obstetricians and Gynecologists. http://www.acog.org/~/media/For%20Patients/faq156.pdf?dmc=1&ts=20121116T1341067069. Accessed Nov. 16, 2012.
  3. Cunningham FG, et al. Williams Obstetrics. 23rd ed. New York, N.Y.: The McGraw-Hill Companies; 2010:1. http://www.accessmedicine.com/content.aspx?aID=6037835. Accessed March 31, 2011.
  4. Moore KL, et al. The Developing Human: Clinically Oriented Embryology. 8th ed. Philadelphia, Pa.: Saunders Elsevier; 2003:4.
  5. Frequently asked questions. Pregnancy FAQ069. What to expect after your due date. The American Congress of Obstetricians and Gynecologists. http://www.acog.org/Resources_And_Publications/~/media/For%20Patients/faq069.ashx. Accessed Nov. 16, 2012.
  6. American College of Obstetricians and Gynecologists. Your Pregnancy and Childbirth: Month to Month. 5th ed. Washington, D.C.: American College of Obstetricians and Gynecologists; 2010:117.
  7. Why breastfeeding is important. The National Women's Health Information Center. http://www.womenshealth.gov/breastfeeding/why-breastfeeding-is-important/#a. Accessed Nov. 16, 2012.
  8. Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. Accessed Nov. 16, 2012.
PR00114 Dec. 4, 2012

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