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Inside the cycle.

Menstruation is rare among mammals: only humans and some other primates, a few bats, elephant shrews and the spiny mouse are known to menstruate.10,11 The rats and mice used in most preclinical studies do not. Instead, they have a short estrous cycle of about 4 to 5 days, in which the uterine lining is reabsorbed rather than shed.10 I built this diagram so engineers can see what changes day to day and spot new ideas for drugs, materials and devices in women's health.

Day 1of 28

Ovary
Uterus

Hormones

EPLHFSH

Hormones

You might notice

Surprising effects

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    Hormone levels across the cycle

    GnRH from the brain tells the pituitary to release FSH (grows follicles) and LH (its surge releases the egg). Follicles make estrogen; the corpus luteum makes progesterone. Curves are relative and show a textbook 28-day cycle.1

    How a hormonal IUD works

    A hormonal IUD is a small, flexible T placed inside the uterus. It slowly releases levonorgestrel, a progestin, mostly right where it sits.4

    • Blocks sperm: it thickens the mucus in the cervix, so sperm can't swim through.
    • Thins the lining: the endometrium stays thin, so periods get lighter or stop and an egg couldn't implant.
    • Slows sperm: it makes it harder for sperm to move and reach an egg.
    • Sometimes stops ovulation: some users stop ovulating, especially early on, but many keep ovulating.

    It is more than 99% effective and works for several years. A copper IUD has no hormones and works differently.4

    Possible risks during this phase

      Most of this research is still mixed, and everyone is different.

      What every organ is doing today
        References
        1. Reed, B. G.; Carr, B. R. The Normal Menstrual Cycle and the Control of Ovulation. In Endotext; Feingold, K. R., et al., Eds.; MDText.com, Inc.: South Dartmouth, MA, 2018. https://www.ncbi.nlm.nih.gov/books/NBK279054/ (accessed 2026-11-06).
        2. McNulty, K. L.; Elliott-Sale, K. J.; Dolan, E.; Swinton, P. A.; Ansdell, P.; Goodall, S.; Thomas, K.; Hicks, K. M. The Effects of Menstrual Cycle Phase on Exercise Performance in Eumenorrheic Women: A Systematic Review and Meta-Analysis. Sports Med. 2020, 50 (10), 1813–1827. DOI: 10.1007/s40279-020-01319-3.
        3. Herzberg, S. D.; Motu’apuaka, M. L.; Lambert, W.; Fu, R.; Brady, J.; Guise, J.-M. The Effect of Menstrual Cycle and Contraceptives on ACL Injuries and Laxity: A Systematic Review and Meta-analysis. Orthop. J. Sports Med. 2017, 5 (7), 2325967117718781. DOI: 10.1177/2325967117718781.
        4. American College of Obstetricians and Gynecologists. Long-Acting Reversible Contraception (LARC): Intrauterine Device and Implant. https://www.acog.org/womens-health/faqs/long-acting-reversible-contraception-iud-and-implant (accessed 2026-11-06).
        5. American College of Obstetricians and Gynecologists. Heavy Menstrual Bleeding. https://www.acog.org/womens-health/faqs/heavy-menstrual-bleeding (accessed 2026-11-06).
        6. Herzog, A. G.; Klein, P.; Ransil, B. J. Three Patterns of Catamenial Epilepsy. Epilepsia 1997, 38 (10), 1082–1088.
        7. Sánchez-Ramos, J. L.; et al. Risk Factors for Premenstrual Asthma: A Systematic Review and Meta-Analysis. Expert Rev. Respir. Med. 2017, 11 (1), 57–72. DOI: 10.1080/17476348.2017.1270762.
        8. Gamarra, E.; Trimboli, P. Menstrual Cycle, Glucose Control and Insulin Sensitivity in Type 1 Diabetes: A Systematic Review. J. Pers. Med. 2023, 13 (2), 374. DOI: 10.3390/jpm13020374.
        9. Raffaelli, B.; et al. Menstrual Migraine Is Caused by Estrogen Withdrawal: Revisiting the Evidence. J. Headache Pain 2023, 24, 131. DOI: 10.1186/s10194-023-01664-4.
        10. Emera, D.; Romero, R.; Wagner, G. The Evolution of Menstruation: A New Model for Genetic Assimilation. BioEssays 2012, 34 (1), 26–35. DOI: 10.1002/bies.201100099.
        11. Bellofiore, N.; Ellery, S. J.; Mamrot, J.; Walker, D. W.; Temple-Smith, P.; Dickinson, H. First Evidence of a Menstruating Rodent: The Spiny Mouse (Acomys cahirinus). Am. J. Obstet. Gynecol. 2017, 216 (1), 40.e1–40.e11. DOI: 10.1016/j.ajog.2016.07.041.

        Educational only, not medical advice. Illustrations by Alexandra L. Ramos Figueroa.