Typically during pregnancy you are constipated, as the baby tends to push everything in your tummy.
With both my pregnancies I had loose bowels. The first time I thought I had an infection, then the second time it happened again however loose bowels with no other signs or symptome especially a missed period is not a sign of pregnancy.
Some women that are about to have their periods can have more frequent bowel movements. Some foods such as leafy veggies can also create this.