As your pregnancy progresses and your belly begins to grow it will push out against your recti abdominal muscle, which are two muscles that run parallel to one another either side of your belly button (known as your six-pack). These muscles cover your tummy and can separate in later stages of pregnancy (late second trimester and third trimester) when many women experience diastasis recti, which is the separation of the recti abdominal muscles. The separation of the muscles can often go unnoticed, although by the last few weeks you may feel discomfort (usually as a stretching or 'splitting' sensation), which generally eases within a few day.
After birth you may be left with a soft gap between the recti abdominal muscles, called diastasis recti. The gap will gradually close, which can take up to one year for some women. There's a separate blog on this, click here.