Luteal Phase

The luteal phase begins after the mid-cycle LH surge has caused ovulation. The follicle that released the oocyte does not simply disappear; its granulosa and theca cells reorganise, enlarge, and become highly vascularised luteal cells. This temporary endocrine gland is the corpus luteum. LH is the principal luteotrophic hormone maintaining the corpus luteum in a non-pregnant cycle. Granulosa-lutein and theca-lutein cells produce mainly progesterone, along with estradiol and inhibin A. Progesterone changes the reproductive tract from an oestrogen-primed state to one that can support implantation: - The endometrium undergoes secretory transformation. Glands become more secretory, and the stroma develops the changes associated with early decidualisation. - Cervical mucus becomes thicker and less penetrable to sperm. - Basal body temperature rises by approximately 0.3–0.5 °C. This is a progesterone effect on hypothalamic temperature regulation, not evidence of infection. - Myometrial contractility decreases, helping maintain uterine quiescence. - Breast tenderness or fullness may occur as ovarian steroid levels change. The luteal phase is the more predictable portion of the cycle and usually lasts about 14 days in a non-conception cycle. The follicular...