The difference between the swelling of the abdomen after pregnancy and the swelling of the abdomen before menstruation lies mainly in checking the basal body temperature. The basal body temperature does not drop when you are pregnant and is usually around 37°C, not lower than 36.8°C but not more than 37.5°C either. If this high temperature persists for more than 15 days, the possibility of pregnancy is very high. If the basal body temperature drops before you get your period, you will get your period about 24 hours after it drops, which is the main difference between bloating and pain in the abdomen when you are pregnant and bloating and pain when you get your period. The vast majority of women experience slight discomfort in the lower abdomen when they are pregnant, or before they have their period, and for this it is difficult to distinguish between pregnancy and menstruation, so it is recommended that the difference be made by a basal body temperature test.