By flooding the uterus with a rich supply of blood for the tissue that lines the inner side of the uterus and it is called the endometrium. This happens each month during the menstrual cycle, when the body with the help of hormones which have different levels in the different times of the cycle. When it is time to prepare the uterus for a fertilized egg, the blood vessels of the endometrium swell and supply more blood to the tissue.
In the case that a fertilized egg cell reaches the uterus, it will attach itself to the endometrium and this is the place where the fetus, and then the baby will start growing. The endometrium is the base for the growth of the placenta which is the main and most important factor for protection and nutrition of the baby.