Week 25

Week 25 of Pregnancy: Baby's Lung Development 

Your baby is about nine inches long and more than a pound and a half in weight this week. And don't look now (as if you could), but your baby's skin is turning pinker as small blood vessels form under the skin and fill with blood. Lungs and nostrils are also growing at this stage of fetal development.

At nine inches and more than a pound and a half in weight, your baby is taller than two juice boxes stacked one on top of the other, and almost as heavy as four of those little boxes (an analogy you'll find particularly apropos in a few years, when those juice boxes start taking over your refrigerator, your pantry, your car.…).
 
Don't look now (as if you could), but your baby's skin is turning pinker. No, not because he or she's getting overheated (in fact, the amniotic fluid is perfectly climate controlled, keeping your baby at an always comfortable temperature), but because small blood vessels, called capillaries, are forming under the skin and filling with blood. Later this week, blood vessels will also develop in your baby's lungs, bringing them one step closer to full maturity — and one step closer to taking that first breath of fresh air. But at 25 weeks pregnant, those lungs are still very much works in progress. Though they are already beginning to develop surfactant, a substance that will help the lungs expand after the baby is born, the lungs are still too undeveloped to sufficiently send oxygen to the bloodstream and release carbon dioxide when he or she exhales.

The lungs aren't the only system that's gearing up for air intake. Your baby's nostrils, which have been plugged up until now, are starting to open this week. This actually allows your little one to begin taking practice breaths. (Of course since there's no air in there, your baby is really only "breathing" amniotic fluid, but it's the practice that counts, right?)