The short answer to your question is that nothing is in the lungs of a baby before it is born.

Before we are born, we get all of our oxygen from our mothers lungs. She breathes in oxygen. It goes into her blood. Some of her blood goes to the placenta where it gets very close to the fetuss blood. They dont actually mix, but some of the oxygen gets transferred to the fetal blood. The fetal blood is stickier for oxygen than the mothers blood is. This blood travels from the fetuss side of the placenta, through the umbilical cord, thorough the fetuss body. While its delivery oxygen, the blood picks up carbon dioxide, a waste gas. It takes this back through the umbilical cord, to the placenta, where it gets picked up by the mothers blood. When her blood gets to her lungs, she breathes out the carbon dioxide.

Thats a lot to read about, so check the picture at:

So what are the lungs doing? Nothing, really. Think of the lungs as being millions of tiny balloons. In a fetus, the balloons are all flat, with no air inside. Even the blood takes a detour around the lungs. Instead of going into the arteries that lead to the lungs, most of the blood is sent to the rest of the body. When the baby is born, it takes its first breath and the lungs inflate like balloons.

How do you think animals inside of eggshells get their oxygen?

Before a baby is born, its lungs are actually filled with fluid! A baby gets all the oxygen it needs from its moms blood. It is not until the baby comes out of the womb that it takes its very first breath of air. Actually, scientists are still trying to figure out how a baby knows to start breathing air when it is born since it has never had to use its lungs like that before.

Baby lungs are filled with water before they are born, just like pretty much everything else in the body. They have to expel the water before they can actually breathe, which is the first thing a baby does when born.