If you lie flat on your back for a few minutes or more during the later months of the pregnancy (after about 24 to 28 weeks of pregnancy), you may start to feel light-headed, dizzy and possibly breathless. This does not happen to every pregnant woman. However, if it does it is because your growing baby is making your uterus heavy, and is placing pressure on one of your major blood vessels called the 'vena cava'. The vena cava lies on the right side of your body. During pregnancy your enlarged uterus also naturally leans towards your right side (as it moves up and out of the pelvis after 12 weeks). This can make the vena cava blood vessel prone to becoming compressed while lying on your back.
Image 2-18 shows how the heavily pregnant uterus can compress the 'vena cava' when the woman lies flat on her back.
The vena cava carries your blood supply back from your body to your heart. If this blood supply is reduced by lying on your back, then this may slow the flow of blood from the heart to other parts of the body (such as your lungs and brain, causing
breathlessness and dizzy sensations). However, generally pregnant women need to lay quite flat on their back before the vena cava becomes compressed. Usually by lying on your right or left side, sitting in an upright position, or even in a semi-upright position will avoid vena-caval compression.
If the woman's blood flow to her heart is reduced, then her unborn baby may also receive less blood flow for a short period of time. This may occasionally be enough to cause the baby to be temporarily stressed and in perhaps open their bowels inside the uterus before the birth. A baby's first bowel motion is called 'meconium', and the medical term given to when a baby passes this before the birth is 'meconium stained liquor' (or 'MSL'). Meconium stained liquor becomes evident if the waters break around the time of labour and the amniotic fluid is stained a greenish colour. You can read more in