If a fertilized egg does not attach to the womb lining by the end of the monthly menstrual cycle, the levels of progesterone in the body decrease. This
causes the body to shed the womb lining (a menstrual ‘period’). If a fertilized egg successfully attaches to the womb lining by the end of the monthly cycle, progesterone levels in the body remain high. This helps maintain a healthy womb lining for the ongoing pregnancy.