Pregnancy 

Feeling short of breath is common in pregnancy. But this could be a warning sign of serious condition. You may be out of breath because of more oxygen demand in pregnancy and your body will adapt to meet this new demand. Progesterone hormone stimulates your respiratory center in your brain. Feeling short of breath in early pregnancy may be due to heightened awareness of the need to breathe. Later in pregnancy, your breathing is laboured from the pressure of your growing womb