At this point in pregnancy, baby's lungs are getting ready for breathing. Airway passages form tubes in order to take in and expel air. Blood vessels and air sacs develop in the lungs; they will eventually exchange oxygen and circulate it to all parts of her body. The cells in the fetal lungs begin to produce surfactant, a substance that keeps the sacs from sticking together. All of this means that your baby's lungs have developed enough so that, conceivably, your little one could survive outside the womb.
Still, your baby still has a lot of growing to do. Optimally, he will stay put until what is considered full term at the end of the 37th week.