No, the thickening of uterus lining is not permanent. It gets thickened to receive the fertilized egg but if the egg is not fertilized the inner lining sheds off. Blood along with mucus, an unfertilized egg is released out through menstrual flow. But if the egg is fertilized the uterus receives the zygote and supplies nourishment through the blood vessels. Till the complete development of the foetus in the uterus it provides nutrients, oxygen to the growing fetus. After the baby is born it sheds off.