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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

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.

26-week 2D ultrasound: Cross-section of baby’s abdomen, measuring approximately 6.7 centimeters (2.5 inches) in diameter.