As the fetus continues to grow inside your uterus, the uterus expands and takes up more room in your abdomen, causing your other organs to be squeezed and shifted. By about 31-34 weeks of pregnancy, the uterus has grown so large that it presses the digestive organs and the diaphragm up toward the lungs. Because this limits the ability of the lungs to fully expand when you take a breath, you may feel short of breath. Despite this uncomfortable feeling, you do not need to worry about your baby getting enough oxygen. The fetus gets all of the oxygen it needs even when you are feeling short of breath. 

If you had mild asthma before you were pregnant, you may find that your asthma symptoms become worse during pregnancy. 

As the fetus moves down in the uterus to prepare for delivery (usually between the 36th and 38th week of pregnancy), the pressure on your lungs will decrease and you will be able to breathe easily again. 

Tips to relieve the uncomfortable feeling of being short of breath: 

• Move a little more slowly so your heart and lungs do not have to work so hard.
• Sit up straight to give your lungs as much room as possible to expand.
• Sleep with your upper body propped up on pillows or in a reclining chair.