A feeling of being out of breath, or “winded”, as if you were in really poor shape, is a condition many women experience when they are expecting. Your pregnancy is increasing the load on all organs. Your body is making a new little human being! As a result, your breathing may be heavier than normal. During pregnancy, at least one liter (one quart) extra blood is produced. This means that your heart has to work faster to pump the blood around your body, which in turn causes your pulse to rise. When your pulse rises, you need to breathe a bit faster.
Yet another reason for breathlessness toward the end of pregnancy is that the baby in your womb is taking up more space from your other organs. Your growing baby pushes your uterus against your diaphragm, prohibiting your lungs from expanding at full capacity.
If you feel extremely out of breath, speak with your midwife or healthcare provider to decide if you need an appointment to rule out something else as the reason why you feel breathless.
Get medical attention immediately if you get so winded that you find it difficult to take deep breaths. A blood clot can, in rare cases, form in the lungs since pregnant women are at higher risk for blood clots.
Source: - Lärobok för barnmorskor. Faxelid, E. Course literature 2001. - Myles Textbook for Midwives. Fraser, D. och Cooper, M. Churchill Livingstone 2003.