This week heralds one of the most important thresholds in pregnancy: Your baby is considered "viable." That means that he now weighs well over 1 pound and has a better than 50 percent chance of surviving with intensive care if you deliver prematurely. This is due in large part to the final development of airway passages and air sacs in his lungs.
Your unborn baby's lungs have been "practicing" breathing for several weeks. Since week 17, he's been taking amniotic fluid into his lungs and expelling it, in breathlike motions. His lungs are now mature enough to take the next step toward taking in air. At week 24, your baby-to-be begins to produce surfactant, an important fluid necessary for your baby to breathe outside the womb.
To understand the importance of surfactant, you must also have some idea about how the lungs develop and function. Lungs extend out like trees -- both sides have a main "trunk" that then branches out into smaller parts. All of these small parts give the lungs greater surface area from which to draw in oxygen, absorb it, and force out carbon dioxide. The smallest parts of the lung, on the ends of the extending "branches," act like little balloons filling with air, then emptying. These small structures within the lungs produce surfactant, a complex fluid that helps air enter and exit the lungs more easily. Surfactant acts as a lubricator. In other words, it decreases the surface tension of the lungs. To explain how this works, think of the lungs as a sieve and air as sand. If the sieve were lined with sandpaper, the sand would move slowly and might even stall on the coarse surface. But if the sieve were made of smooth, oiled plastic, the sand would pass through quickly.
Along with development in your baby-to-be's lungs, he's continuing to add fat tissue. During the remaining weeks of pregnancy, your baby will gain about 6 to 7 pounds. His ears are now extremely sensitive to sounds outside the womb, and he can recognize your voice. In fact, research shows us that babies have their earliest language lessons in the womb; after birth, they suck more vigorously if they hear tape recordings of people speaking in their native tongue than they do if they hear people speaking a foreign language. There is no evidence that babies prefer the voices of their fathers, grandmothers, or siblings, but they definitely recognize their mothers' voices and prefer them once they're born.
Through ultrasound, scientists have learned that even unborn babies love to play. Yours will continue to pedal around on the uterine walls, and he'll try to grab everything around him with his increasingly strong grip. His favorite toy -- well, his only toy, really -- is the umbilical cord, which he might yank or swing or pull on for entertainment. Don't worry, though. This thick bundle of blood vessels is remarkably strong, and the force of blood rushing through it keeps the cord extended and prevents kinking.
Surfactant: A fluid produced by the alveoli and bronchioles of the lungs to aid in oxygen passage into and out of the body.
Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).