This is a randomized, double-blind placebo-controlled trial of oral micronized progesterone (300 mg daily at bedtime) for perimenopausal women living anywhere in Canada. Using the self-reported maximum menstrual cycle length in the previous year, women will be stratified as in Early Perimenopause (<60 days) or Late Perimenopause (>=60 days). The design includes a 28-day baseline run-in followed by 12 weeks of randomized therapy.