Wiktionary
follicular phase
n. (context physiology English) The phase of the estrous cycle which involves the follicular maturation within the ovary and is controlled by the hormone estradiol.
Wikipedia
Follicular phase
The follicular phase is the phase of the estrous cycle, (or, in humans and great apes, the menstrual cycle) during which follicles in the ovary mature. It ends with ovulation. The main hormone controlling this stage is estradiol.