Conception Day Gender Calendar

How each number is worked out

Short version, so you can judge how much weight to give each column.

The fertile window (the part that is real)

Ovulation is placed by counting backwards from the next expected period, not forwards from the last one — the luteal phase is the stable half of the cycle. So:

• Next period = last period start + cycle length (repeated forward through the range)
• Ovulation = next period − luteal phase, i.e. day (cycle length − luteal) counting day 1 as the first day of bleeding
• Fertile window = ovulation − 5 days through ovulation + 1 day

This is calendar arithmetic on an average cycle, so it shifts if the real cycle is longer or shorter that month. Cervical mucus observation, basal body temperature, or an LH ovulation test will always beat a prediction on paper.

The non-fertile days

Every day in the calendar now carries its cycle day, its phase and a "chance" rating, not just the days inside the fertile window:

• Period (red) — day 1 onward, for as many days as you set. Chance: very low.
• Follicular (plain) — bleeding has stopped, the fertile window hasn't opened. Chance: very low, rising to low in the two days before the window, because ovulation can arrive earlier than an average predicts.
• Fertile (green) — high, and peak on ovulation day itself.
• Luteal (violet) — after ovulation. Chance: low for two or three days, then very low. The egg is unfertilisable roughly 24 hours after release, so intercourse here cannot cause this cycle's conception; the "low" rating covers the possibility that ovulation actually happened later than predicted.

The two gender predictions are shown on these days too, so you can see what each method would say about any date. Worth keeping in mind that a prediction for a day in the luteal phase is doubly hypothetical: conception almost certainly cannot happen then, and the method behind the prediction doesn't work anyway.

Method 1 — The Chinese lunar chart

A grid attributed to a Qing-dynasty archive that cross-references two numbers:

• The mother's Chinese lunar age (虚岁, xūsuì) — you are 1 at birth and gain a year at each Chinese New Year, not on your birthday. This page computes Chinese New Year properly from the new moon following the winter solstice, rather than approximating it with 1 January.
• The Chinese lunar month of conception — also computed properly: months run new moon to new moon, month 11 is the one containing the winter solstice, and leap months are detected by the "no major solar term" rule. Because lunar months straddle Gregorian ones, the chart's answer can change part-way through a calendar month. That is why each month header lists the lunar months it overlaps.

Ages 18–45 are covered by the chart. Outside that range the "Ch" pill is left blank.

Method 2 — The Jonas method

Dr. Eugen Jonas, a Slovak psychiatrist, proposed in the 1950s that a woman has a second fertility rhythm locked to the Moon's phase at her own birth, and that the Moon's zodiac sign at conception determines the baby's sex.

• Gender — the Moon's tropical zodiac sign each day. Fire and Air signs (Aries, Gemini, Leo, Libra, Sagittarius, Aquarius) are read as boy; Earth and Water signs (Taurus, Cancer, Virgo, Scorpio, Capricorn, Pisces) as girl. The Moon changes sign roughly every 2.5 days, so the prediction flips every few days.
• Peak day (★) — the angle between Sun and Moon at the mother's exact birth moment is calculated, then the calendar is scanned for the day the same angle recurs. This happens about once every 29.5 days.

Sun and Moon positions use a reduced Meeus algorithm (about 49 periodic terms for the Moon), accurate to well under a tenth of a degree — far finer than the method itself warrants, but it means the sign changes and peak days land on the right day.