Women often complain of breathlessness during the last trimester, though some women experience it throughout their entire terms.
It is caused by the enlarging uterus which pushes the abdominal content upwards underneath the ribs. The uterus pushes the diaphragm into the chest which in turn constricts respiratory movement of the chest. When you exhale, your diaphragm cannot descend after a point causing inefficient breathing and shortness of breath at the slightest exertion.
You may feel breathless at night if you lie flat on your back. Though this problem cannot be solved entirely, you could try propping your head and shoulders up on a couple of pillows which reduces the pressure of the enlarging uterus on the diaphragm.
Breathlessness in pregnancy usually does not interfere with day to day activity, but if it becomes much too uncomfortable, report to your doctor.
It may also get reduced by the end of 36 weeks as the baby’s head descends to the bony pelvis, causing the upper part of the uterus to descend about five to seven centimetres. This, in turn, relieves the pressure on the diaphragm.