How does the menstrual cycle determine the fertility window?

The menstrual cycle lasts 28 days on average and is divided into four phases. Ovulation, triggered by a surge of LH hormone, releases an egg cell around day 14. This egg is only viable for 12 to 24 hours, but sperm can survive up to 5 days in the reproductive tract. The fertility window therefore spans about 6 days around ovulation. Progesterone, secreted after ovulation, prepares the uterus for a potential implantation.

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The menstrual cycle: the science of fertility

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In a conversation about the courtesans of the pleasure district, it’s mentioned that a courtesan who knows her cycle can estimate when she will be most fertile. The line is brief, almost offhand, but it says something important about how practically knowledgeable these women were: no molecular biology, no hormone testing, just an empirical observation of the body precise enough to identify a risky window or, depending on the circumstances, a sought-after one. In an imperial court setting where bearing an heir determined one’s rank, this knowledge carried considerable value.

The subject in depth

The menstrual cycle is the process by which the female body prepares each month for a potential pregnancy ^[Wikipedia]. Its length varies from 21 to 35 days depending on the individual, with an average of 28 days often used as a teaching reference.

Diagram of the four phases of the menstrual cycle, with estrogen and progesterone curves — The menstrual cycle

The four phases. The cycle begins with menstruation (the menstrual phase, roughly days 1 to 5): the uterine lining built up during the previous cycle is shed. Next comes the follicular phase: driven by FSH (follicle-stimulating hormone), several ovarian follicles begin maturing in the ovary. One becomes dominant and secretes increasing amounts of estrogen, which rebuilds the uterine lining and prepares the cervix. Around day 12 to 14, estrogen levels peak, triggering a sharp surge of LH (luteinizing hormone), the direct signal for ovulation ^[Wikipedia]. The ovary releases the egg: this is ovulation. The luteal phase then begins, during which the ruptured follicle turns into the corpus luteum and secretes progesterone. This hormone thickens the uterine lining to favor implantation. Without fertilization, the corpus luteum breaks down after about ten days, estrogen and progesterone levels drop, and menstruation resumes.

The fertility window. The released egg is only viable for 12 to 24 hours. Sperm, on the other hand, can survive in the female reproductive tract for up to five days. The fertility window therefore spans about six days: the five days before ovulation plus the day of ovulation itself. This is what gynecologists Kyusaku Ogino (1924, Japan) and Hermann Knaus (1929, Austria) independently formalized, giving rise to the Ogino-Knaus method, the first attempt at natural contraception based on cycle tracking.

The role of hormones. Estrogen dominates the first half of the cycle: it stimulates follicular growth, thickens the uterine lining, and thins the cervical mucus to ease sperm passage. Progesterone takes over after ovulation: it makes the lining receptive, thickens the mucus to block further sperm, and sustains pregnancy if implantation occurs. The two hormones work in tandem, each dominating one half of the cycle.

In real life

Empirical knowledge of the female cycle is ancient across many cultures. Classical Chinese medicine linked menstruation to qi and blood, treating its regularity as an indicator of a woman’s overall vitality. Lunar cycles served as a universal reference point: in China, in India, and in the medieval Arab-Muslim world, several medical texts tied the menstrual rhythm to the phases of the moon, a crude but roughly consistent approximation given the average 28-day length. Hippocratic and Galenic Greek medicine, which influenced Europe until the 17th century, interpreted menstruation as a purge of impure blood and had no concept equivalent to ovulation. The egg cell itself was not observed until 1827, by Karl Ernst von Baer. Yet practical observations about the fertile period circulated across several traditions: the Babylonian Talmud (5th-6th century) contains timing recommendations for intercourse, and some Ayurvedic medical texts describe favorable days for conception in calendrical terms. This empirical knowledge, passed down from practitioner to practitioner rather than through officially recognized treatises, represents a form of observational research spanning many generations.

The fact-checker's eye

The show’s line is scientifically honest. Claiming that a woman can estimate her fertile period by knowing her cycle matches a real biological fact, confirmed by the Ogino-Knaus method and its modern variants (the symptothermal method, LH-surge tracking). The shortcut lies in precision: the cycle is not a perfect clock, and ovulation can shift from one cycle to the next under the effect of stress, nutrition, or illness. For reliable contraception, this variability makes calendar-based methods unsafe without complementary measuring tools. In the story’s world, set in a context resembling imperial China, the absence of hormone tests or precise thermometers made this empirical knowledge both valuable and imperfect. The work does not claim more than that: it suggests an estimate, not a certainty.

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Published on May 30, 2026

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