A 28-year-old woman from Delhi presents with irregular menstrual cycles. She reports that her periods come every 35–45 days, with cycle lengths varying unpredictably. Menarche was at age 13, and she has never been pregnant. Physical examination is unremarkable; BMI is 22 kg/m². Pelvic ultrasound shows normal uterus and ovaries with multiple small follicles. FSH is 6 mIU/mL, LH is 8 mIU/mL, and prolactin is normal. Which phase of the menstrual cycle is most likely prolonged in this patient?