--- title: "Follicular Phase | General Biology I" description: "Follicular phase in General Biology I: the menstrual cycle stage when ovarian follicles mature, estrogen rises, and the uterine lining rebuilds before ovulation." canonical: "https://fiveable.me/college-bio/key-terms/follicular-phase" type: "key-term" subject: "General Biology I" unit: "Unit 43" --- # Follicular Phase | General Biology I ## Definition The follicular phase is the part of the menstrual cycle in General Biology I when ovarian follicles mature under FSH, estrogen rises, and the endometrium rebuilds before ovulation. ## What It Is The follicular phase is the first half of the menstrual cycle in General Biology I, starting on day 1 of menstruation and ending at ovulation. During this time, several ovarian follicles begin to grow, but usually only one becomes the dominant follicle that will release an egg. The main signal driving this phase is follicle stimulating hormone, or FSH, released by the anterior pituitary gland. FSH stimulates the ovarian follicles to develop and encourages the cells around the egg to make estrogen. As estrogen levels rise, the body shifts from shedding the old uterine lining to rebuilding it. That rebuilding happens in the endometrium, the lining of the uterus. Rising estrogen causes the endometrium to thicken and become more ready for a possible embryo. This is why the follicular phase is sometimes described as the preparatory phase of the cycle, even though the egg has not been released yet. Not every follicle that starts growing survives. Many undergo atresia, which means they stop developing and degenerate. The dominant follicle keeps responding to hormones and grows faster than the others, which helps set up the next step in the cycle. Near the end of the follicular phase, the dominant follicle makes enough estrogen to trigger a surge in luteinizing hormone, or LH, from the pituitary. That LH surge is the switch that ends the follicular phase and starts ovulation. If you are tracing the cycle, think of the follicular phase as the growth-and-build phase before the egg is released. ## Why It Matters The follicular phase is the part of the menstrual cycle that connects hormone signaling to visible body changes, so it shows up a lot in General Biology I when you trace endocrine feedback. It lets you follow the chain from the hypothalamus and pituitary to the ovaries and then back to the uterus. This term also helps explain how estrogen affects more than just reproduction. As follicle cells make more estrogen, the hormone changes the endometrium and feeds back to the brain and pituitary. That feedback is what eventually helps trigger the LH surge, which is easy to miss if you only memorize hormone names without following the sequence. If you are comparing stages of the cycle, the follicular phase is the contrast point for the luteal phase. One phase is building toward ovulation, while the other follows ovulation and centers on the corpus luteum. Knowing which hormones are high, which structure is dominant, and what the uterus is doing makes diagrams, timelines, and hormone graphs much easier to read. ## Connections ### [Ovarian Follicles](/college-bio/key-terms/ovarian-follicles) The follicular phase gets its name from the follicles that grow inside the ovaries during this stage. These structures contain the developing egg and the supporting cells that respond to FSH. When you see a cycle diagram, the follicular phase is the time when follicle growth is the main ovarian event, before one follicle becomes dominant. ### follicle stimulating hormone (FSH) FSH is the pituitary hormone that starts the follicular phase by stimulating follicle growth. Without FSH, the follicles would not mature normally and estrogen production would stay low. In questions about hormone order, FSH is usually the signal that comes before the estrogen rise and before the LH surge. ### Estrogen Estrogen levels rise during the follicular phase as the developing follicles grow. That rise causes the endometrium to thicken and also helps create feedback on the pituitary. If you are tracking a hormone graph, a strong estrogen increase near the end of this phase is what helps set up the LH surge. ### [luteal phase](/college-bio/key-terms/luteal-phase) The luteal phase comes after ovulation, so it is the phase that follows the follicular phase. The follicular phase ends when the LH surge releases the egg, and the luteal phase begins as the corpus luteum forms. Comparing the two helps you separate follicle growth from post-ovulation hormone production. ## On the AP Exam A quiz question may ask you to place the follicular phase in order, identify which hormone rises, or match it to the correct ovarian and uterine changes. You might also see a graph where FSH rises first, estrogen climbs as the follicle matures, and LH spikes at the end. The move is to connect the hormone pattern with the structure that is changing. If the question shows a cycle diagram, look for the stage between menstruation and ovulation, when follicles develop and the endometrium thickens. On short-answer items, use the term to explain why one follicle becomes dominant and how that sets up ovulation. ## Follicular Phase vs luteal phase The follicular phase happens before ovulation, while the luteal phase happens after ovulation. Follicular phase means follicle growth, rising estrogen, and endometrial rebuilding. Luteal phase means the corpus luteum is active and progesterone is the main hormone. If you mix them up, check whether the question is asking about pre-ovulation or post-ovulation events. ## Key Takeaways - The follicular phase is the first part of the menstrual cycle, running from menstruation to ovulation. - FSH from the anterior pituitary stimulates ovarian follicles to grow and mature. - Developing follicles secrete estrogen, which thickens the endometrium and helps prepare the uterus for a possible embryo. - Usually one dominant follicle survives while the others undergo atresia. - A surge in LH ends the follicular phase and triggers ovulation. ## FAQs ### What is follicular phase in General Biology I? It is the first half of the menstrual cycle, when ovarian follicles mature under FSH and estrogen rises. In this phase, the uterus also rebuilds its lining after menstruation. The phase ends when the LH surge triggers ovulation. ### Is the follicular phase before or after ovulation? It happens before ovulation. The follicular phase starts with menstruation and ends right when the mature egg is released. After ovulation, the cycle moves into the luteal phase. ### What hormones are involved in the follicular phase? FSH starts the phase by stimulating follicle growth, and the follicles then produce estrogen. Near the end of the phase, rising estrogen helps trigger the LH surge. That LH spike is what leads to ovulation. ### How is the follicular phase different from the luteal phase? The follicular phase is about follicle growth and estrogen production before ovulation. The luteal phase comes after ovulation and centers on the corpus luteum, which produces progesterone. If you are reading a cycle chart, the big clue is whether the egg has been released yet. ## Related Study Guides - [43.4 Hormonal Control of Human Reproduction](/college-bio/unit-43/4-hormonal-control-human-reproduction/study-guide/GZ47Ra929ziQkm4Q) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/follicular-phase#resource","name":"Follicular Phase | General Biology I","url":"https://fiveable.me/college-bio/key-terms/follicular-phase","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/follicular-phase#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:09.279Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/follicular-phase#term","name":"Follicular Phase","description":"The follicular phase is the part of the menstrual cycle in General Biology I when ovarian follicles mature under FSH, estrogen rises, and the endometrium rebuilds before ovulation.","url":"https://fiveable.me/college-bio/key-terms/follicular-phase","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is follicular phase in General Biology I?","acceptedAnswer":{"@type":"Answer","text":"It is the first half of the menstrual cycle, when ovarian follicles mature under FSH and estrogen rises. 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