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Phase calculator

Estimate your current menstrual cycle phase from the first day of your last period and your usual cycle length.

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Understanding your menstrual cycle: a complete guide to the 4 phases

Let's explore the four distinct phases of the menstrual cycle and discover what's really going on beneath the surface.

The 4 phases of your menstrual cycle

Phase 1: Menstruation (Days 1-5)

What's happening: This is day one of your cycle, marked by the start of your period. Your uterus sheds its lining because no pregnancy occurred during the previous cycle. Meanwhile, your ovaries are already preparing the next batch of eggs.

Phase 2: Follicular phase (Days 1-13)

What's happening: The follicular phase actually overlaps with menstruation and continues until ovulation. During this time, your pituitary gland releases follicle-stimulating hormone (FSH), which signals your ovaries to prepare 5-20 eggs, though typically only one will fully mature.

Phase 3: Ovulation (Days 14-16)

What's happening: This is the main event of your cycle. A surge in luteinizing hormone (LH) triggers the release of a mature egg from one of your ovaries.

Phase 4: Luteal phase (Days 15-28)

What's happening: After releasing an egg, the follicle transforms into something called the corpus luteum, which produces progesterone to support a potential pregnancy.

  • Reed, B. G., & Carr, B. R. (2018). The Normal Menstrual Cycle and the Control of Ovulation. In Endotext. MDText.com, Inc.
  • Vigil, P., Cortés, M. E., & Zúñiga, A. (2022). The menstrual cycle and its relationship with the hypothalamic-pituitary-ovarian axis. Frontiers in Endocrinology, 13.
  • Mumford, S. L., et al. (2013). The utility of menstrual cycle length as an indicator of cumulative hormonal exposure. Journal of Clinical Endocrinology & Metabolism, 97(10).
  • Hampson, E., & Young, E. A. (2008). Methodological issues in the study of hormone-behavior relations in humans. Sex Differences in the Brain: From Genes to Behavior. Oxford University Press.
  • Bull, J. R., et al. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digital Medicine, 2(1), 83.
  • Mihm, M., Gangooly, S., & Muttukrishna, S. (2011). The normal menstrual cycle in women. Animal Reproduction Science, 124(3-4), 229-236.

Frequently asked questions

Progesterone is the dominant hormone during the luteal phase. After ovulation, the corpus luteum (the remnant of the follicle that released the egg) produces high levels of progesterone to prepare and maintain the uterine lining for potential pregnancy. Progesterone is responsible for many premenstrual symptoms like bloating, breast tenderness, and mood changes. If pregnancy doesn't occur, progesterone levels drop sharply, triggering menstruation.

In a typical 28-day cycle, the phases last approximately: menstruation (days 1-5), follicular phase (days 1-13, overlapping with menstruation), ovulation (days 14-16, lasting about 12-24 hours), and luteal phase (days 15-28). However, cycle length varies between women and can range from 21 to 35 days. The luteal phase is usually the most consistent at 12-14 days, while the follicular phase can vary more significantly, affecting overall cycle length.

While unlikely, it is possible to get pregnant during your period, especially if you have a shorter menstrual cycle or longer periods. Sperm can survive in your body for up to 5 days, so if you ovulate shortly after your period ends, sperm from intercourse during menstruation could still be viable. This is more common in women with 21-24 day cycles where ovulation occurs earlier. If you're not trying to conceive, it's important to use contraception throughout your entire cycle.

Important notice

This calculator is provided for informational and educational purposes only. It is not intended to diagnose, treat, cure, or prevent any medical condition, nor is it a substitute for professional medical advice, diagnosis, or treatment.

Always consult with a qualified healthcare provider before making any health-related decisions or if you have questions about your health. Never disregard professional medical advice or delay seeking it because of information provided by this tool.

Your use of this calculator is at your own risk. Results are estimates based on general guidelines and may not reflect your individual circumstances.