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Third Trimester Infant Development
A look at how your baby changes from weeks 26 to 40
by: Dr. Peter Doubilet and Dr. Carol Benson
Third Trimester
- During the first six weeks of the third trimester, your baby will more than double in size, growing from approximately 1.5 to 3.5 pounds.
- Your baby's sense of hearing is improving and he can hear your voice and your partner's as you converse. You can feel, and see on ultrasound, his startled reaction in response to a sudden loud noise.
- Your baby gains between a third and a half a pound a week through most of the third trimester (you will probably gain close to a pound a week).
- At 37 weeks, your baby is considered to be full term, even though you may not deliver for a few more weeks.
s you begin the third trimester, you're entering the homestretch of your pregnancy–literally. Your belly is getting bigger by the week and this added size (possibly combined with trouble sleeping and Braxton-Hicks contractions), may be causing you some discomfort. Your baby continues to put on weight and his organs will finish maturing during the third trimester. Here's a detailed look at his development in the final three months:
Weeks 26 to 32
During the first six weeks of the third trimester, your baby will more than double in size–growing from approximately 1.5 to 3.5 pounds. He will also grow in length from about 14 inches to almost 17 inches. The baby's growth during this period is an important indicator of his health. Using ultrasound, the baby's head, abdomen, and femur (thighbone) can be measured to calculate his approximate weight and predict what his size will be at birth.
Your baby's sense of hearing is improving and he can hear your voice and your partner's as you converse. You can feel, and see on ultrasound, the baby's startled reaction in response to a sudden loud noise. You can also see the baby's eyes move around in a pattern that is similar to rapid-eye-movement (REM) sleep.
His lungs are also more developed, although not yet fully matured. A baby born during this time will usually be fine–although he may need a little help breathing at first. Your baby's bones are fully developed now, even though they are still soft and pliable. He is also gaining muscular strength, so you may notice his movements are becoming more forceful.
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26-week 2D ultrasound: Cross-section of baby’s abdomen, measuring approximately 6.7 centimeters (2.5 inches) in diameter. |
Weeks 33 to 36
During this time your baby continues his rapid weight gain, gaining about half a pound per week (mom will usually gain about a pound a week). By week 36, the average baby will weigh almost six pounds and measure close to 19 inches in length.
The lanugo–a thin layer of hair that covers your baby's body–starts to fall off and is replaced by vernix, which is a white protective coating. Your baby may suck his thumb and his pupils can now constrict and detect light.
Your baby's growth and health still depend on the nourishment and oxygen provided by the placenta, carried to him through the umbilical cord. During this time, your doctor can check on the placenta and the umbilical cord by using a special form of ultrasound, called Doppler, to assess the blood flow to the baby. The pattern of blood flow in the umbilical cord indicates how well the placenta is nourishing the baby.
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36-week 3D ultrasound: The baby is filling out and has more fat on his face and other features. |
Weeks 37 to Term
Once you reach 37 weeks of pregnancy you are considered to be at full term, though delivery may not occur for several more weeks. Your due date is assigned at 40 weeks, although very few women give birth on the exact date. By 40 weeks, the average birth weight of babies in the United States is almost 7.5 pounds. The baby will measure about 20 inches in length.
During this time, your baby continues to put on weight and his organ function will continue to improve. Ultrasound may be used if your doctor is uncertain as to which way the baby is positioned–head down or breech. If you don't go into labor on your own, you will be induced between 41 and 42 weeks.
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38-week 3D ultrasound. The baby is smiling–perhaps he knows he’ll soon get to see the outside world. |
Meet our experts:
Peter Doubilet, MD, PhD and Carol B. Benson, MD are professors of radiology at Harvard Medical School and obstetrical ultrasound specialists at the Brigham & Women’s Hospital in Boston. You can learn much more about pregnancy through the “eyes” of ultrasound in Peter and Carol’s new book Your Developing Baby (McGraw-Hill, 2008). It offers an illuminating “tour” of life in-utero using remarkable 2D and 3D ultrasound images, original explanatory diagrams and reader-friendly prose.
