What are the changes that take place during the menstrual cycle from day 1-4 to unfertilized egg in ovary?

During days 1-4 of the menstrual cycle, menstruation occurs as the uterine lining sheds due to the decline of hormones like estrogen and progesterone from the previous cycle. As the cycle progresses, the follicles in the ovaries begin to develop under the influence of follicle-stimulating hormone (FSH). By the end of this period, one follicle becomes dominant and continues to mature, while others undergo atresia. An unfertilized egg remains in the dominant follicle, and as estrogen levels rise, the uterine lining begins to thicken in preparation for a potential pregnancy.