Chinese Gender Calendar by LMP: How to Convert Your Last Period Date Into a Prediction

The Chinese gender calendar is a traditional Qing dynasty chart that cross-tabs two inputs to give a boy-or-girl guess: the mother’s Chinese lunar age at the time of conception, and the Chinese lunar month in which conception happened. The chart itself does not have an LMP column. It has a conception column. So using LMP means doing one small conversion before the lookup — not throwing out the LMP date, but translating it.

In the OB world, LMP is the official zero point of pregnancy. Per the American College of Obstetricians and Gynecologists, a pregnancy is dated from the first day of the last menstrual period, even though no one is actually pregnant on that day. This convention exists because LMP is observable and verifiable, whereas the day of conception is, for most couples, a guess.

That convention is also the reason most women searching for “chinese gender calendar by LMP” cannot just type their LMP into the chart and get an answer. They need the conception lunar month. The good news: you can get there in two steps, and once you understand the logic, you do not have to trust a black box.

Obstetric dating starts from LMP for a practical reason: it is the only event in early reproductive life with a date attached to it. A typical menstrual cycle runs about 28 days, ovulation tends to occur near the middle of that cycle, and a pregnancy lasts roughly 280 days (40 weeks) from LMP. Conception itself happens around day 14, so a pregnancy is closer to 266 days of actual fetal development — but the calendar people use everywhere from prenatal apps to insurance paperwork counts the full 280-day version.

That is why your due date is 40 weeks after LMP and not 38 weeks after conception. The two are arithmetically the same pregnancy, just measured from two different reference points. Researchers indexed in the National Institutes of Health literature on pregnancy dating note that LMP-based estimates have known variability — cycle length differences, ovulation timing, late or unrecognised implantation bleeding — which is why first-trimester ultrasounds are used to refine due dates when the two disagree.

For the Chinese gender calendar, however, the chart is forgiving. Each lunar month spans roughly 29 to 30 days, so as long as your LMP-to-conception estimate lands inside the correct lunar month, the lookup result is unchanged. The conversion does not have to be perfect — it just has to land in the right month.

The conversion is short. The longest step is finding the LMP itself, which many women have written down already from their first prenatal visit. The math assumes a 28-day cycle; cycle adjustments are covered in step 4.

1. Write down the first day of your last menstrual period. This is day 1 of the cycle in which you conceived. Use the day spotting or full flow began, not the day it ended. If you log periods in an app, this is the date the app marks as cycle start.
2. Add 14 days. Ovulation occurs around the middle of a standard 28-day cycle, and conception happens within roughly 24 hours of ovulation. So LMP + 14 days is the estimated conception date for a typical cycle.
3. Convert that conception date into a Chinese lunar month. The Chinese lunar calendar starts each month on a new moon, so Western calendar dates and lunar months do not align cleanly. Use a lunar conversion tool, or let the calculator on the homepage do it — you will get a lunar month number from 1 through 12.
4. Adjust for irregular cycle length, if needed. The luteal phase (ovulation to next period) is fairly fixed near 14 days, while the follicular phase varies. For a 30-day cycle, use LMP + 16 days. For a 32-day cycle, LMP + 18 days. For a 35-day cycle, LMP + 21 days. Subtract 14 from your typical cycle length to get the offset.
5. Find your lunar age at conception. Chinese lunar age is generally one year older than your Western age (you are “1” at birth) and ticks up at Chinese New Year, not on your birthday. See our lunar age calculator for the exact value at your conception date.
6. Look up the chart cell. Find the row matching your lunar age at conception, then the column matching the lunar month of conception. The cell reads “Boy” or “Girl.”

That is the whole procedure. The two facts most worth memorising are: ovulation is roughly 14 days before the next period, not 14 days after the last one (for cycles longer than 28 days these are different), and the chart cares about lunar months, not Western calendar months.

The table below runs four sample LMP dates through the full conversion. All examples assume a standard 28-day cycle, so conception = LMP + 14 days. The lunar month column shows which Chinese lunar month contains the estimated conception date, using the Hong Kong Observatory lunar almanac as the reference for 2026 lunar boundaries.

Notice the January example: a Western-January LMP can produce a conception date that still falls in the previous lunar year, because Chinese New Year 2026 lands on February 17. This is the most common conversion error people make on their own — the Western calendar year and the Chinese lunar year do not start on the same day, and conception in late January or early February can fall on either side of that boundary. The calculator handles this automatically.

Once you have an estimated conception date, the last step is identifying the Chinese lunar month that contains it. Each lunar month begins on the day of a new moon and lasts either 29 or 30 days, and the months are numbered 1 through 12 (with the occasional leap month in years that need one to stay aligned with the seasons). The lunar new year — lunar Month 1, day 1 — falls between late January and mid-February on the Western calendar each year.

In practice, you do not need to memorise lunar boundaries. You only need a reliable lookup tool. The homepage calculator on the Chinese Gender Calendar homepage accepts a Western conception date and returns the lunar month automatically, using the same astronomical tables that traditional Chinese almanacs use. If you prefer to start from a due date instead, see the dedicated walkthrough on using the Chinese gender calendar by due date, and for the underlying lunar-conception framing see our guide to the Chinese conception calendar.

One subtle point worth flagging: the Chinese lunar month of conception is the lunar month that contains the conception date, not the lunar month in which the mother first noticed missed period symptoms. People sometimes try to guess the lunar month by working backward from when their pregnancy test turned positive, but that lands you about a month late. Always convert from the estimated conception date itself.

The 14-day rule is a good default for the average reproductive-age woman with a regular cycle. There are several situations where it can be off by a week or more, and in those cases a first-trimester ultrasound dating result is more trustworthy than LMP arithmetic. Watch for these:

• Irregular cycles. Cycle lengths under 24 days or over 35 days, or wide variation from one cycle to the next, throw off the LMP + 14 estimate. The luteal phase stays near 14 days, but ovulation can land anywhere from day 10 to day 24 of the cycle.
• Breastfeeding while conceiving. Lactational amenorrhea and the return of ovulation while still nursing can produce cycles where the first ovulation precedes the first “real” period, making LMP an unreliable reference.
• Recent hormonal birth control. The first few cycles after stopping the pill, the patch, the ring, or removing an IUD often run long or irregular as the cycle recalibrates. LMP dating in these months tends to underestimate gestational age.
• Perimenopause. Women conceiving in their early forties sometimes have unpredictable cycles, missed cycles, or breakthrough bleeding that is mistaken for LMP. Ultrasound dating is strongly preferred in this group.
• Implantation bleeding mistaken for a period. Light bleeding around the time of implantation (about 6 to 12 days after conception) can be misread as an unusually light period, putting LMP a full month off.
• Conception during a known late ovulation. Stress, travel, illness, or shift work can delay ovulation by a week or more in an otherwise regular cycle. If you know you ovulated late from OPK tracking or basal body temperature charting, use that date directly instead of LMP + 14.

If any of these apply, the safer move is to either use a doctor-confirmed ultrasound dating result or simply check both lunar months adjacent to your estimate — the one your LMP-based math points to and the one immediately before or after. If the chart prediction is the same for both, the question resolves itself. If it differs, ultrasound dating breaks the tie. To learn more about the human behind this guide, see the Sukie Chinese author page.