23 weeks

Your baby's lungs are in major growth mode this week, with blood vessels expanding and preparing to help her breathe after she's born. Although she's making breathing movements in the womb (so cute!), it's still just practice with amniotic fluid; your baby will get all of her oxygen delivered from the placenta until she's born. But here's why this lung development is so fabulous: If your child should happen to arrive early (though only 2 percent of babies in the U.S. are born before 32 weeks), her odds of surviving are significantly better now that her lungs are up and running. Of course, these and other important organs still have a long way to go to become fully developed, but most experts count weeks 23 and 24 as important, reassuring milestones when it comes to long-term healthy outcomes from preterm labor.