Cramping is normal in early pregnancy, as long as it is not persistent or severe, according to About.com. During early pregnancy, the uterus grows and stretches to accommodate a rapidly growing fetus, causing light to moderate cramping in the belly or lower back.
Around the time of implantation, some women also feel light cramping and even experience spotting or bleeding, according to WebMD. Severe cramping, cramping with heavy bleeding and cramping that seems to be on only one side of the belly are reasons for concern, according to About.com. These symptoms can be signs of a miscarriage or an ectopic pregnancy, which require immediate medical treatment.