If pregnancy does occur, the fertilized egg attaches to the uterus. It is
during the time in between fertilization when the uterus contracts.
During early pregnancy, the uterus will first contract in attempts to dispel the embryo, and then produce growing pains while it extends to make room for the embryo, and cramp-like sensations are felt. In the case of a non-pregnant cycle, after ovulation occurs, the uterine lining is ready to be expelled, resulting in cramps.
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