Hong Kong
Hong Kong Due date calculator
Pregnancy is dated from the first day of your last period, not from the day you conceived, which catches a lot of people out. Enter that date and a typical cycle length and the tool returns an estimated due date, how many weeks and days along you are today, the trimester you are in, and a rough conception date. If you happen to know the day you conceived instead, switch the mode and it counts forward from there. The result follows the same 40-week convention midwives and hospitals use, so the number you see here lines up with the one written in your notes.
How it works
- Pick how you want to date the pregnancy: from your last period (the usual way) or from a known conception date.
- For the last-period method, enter the first day of bleeding and your average cycle length. A 28-day cycle is the textbook default.
- The due date is set 40 weeks (280 days) from that first day, then nudged for a cycle longer or shorter than 28 days.
- The conception method instead counts 38 weeks (266 days) forward from the date you give.
- Alongside the date you get your current progress in weeks and days and which of the three trimesters that falls in.
due date = first day of last period + 280 days + (cycle length - 28)
Naegele's rule sets birth at 280 days, or 40 weeks, after the first day of the last period for a standard 28-day cycle. Because conception happens about two weeks into the cycle, a longer or shorter cycle shifts ovulation and therefore the finish line by the same number of days, which is the cycle correction. The conception method skips that adjustment and counts 266 days, 38 weeks, from the conception date itself.
- LMP
- first day of the last menstrual period
- 280
- days in a 40-week term pregnancy
- cycle - 28
- day-for-day correction for your cycle length
- 266
- days from conception to birth, used in the conception mode
Pregnancy milestones by week
| End of first trimester | 13 weeks | risk of miscarriage drops sharply |
| Anomaly scan | 18 to 21 weeks | detailed mid-pregnancy ultrasound in the UK |
| Viability milestone | 24 weeks | legal limit for most abortions in Great Britain |
| Full term | 37 to 42 weeks | the normal window for a healthy birth |
Worked example
A last period starting 1 March 2026 with a regular 28-day cycle: the estimated due date is 6 December 2026, and conception is placed around 15 March. Stretch the cycle to 32 days and the date moves four days later, to 10 December, because ovulation, and so conception, happens later in a longer cycle.
Key facts
- A pregnancy is counted as 40 weeks but actually lasts about 38 weeks from conception, since the clock starts two weeks early.
- Fewer than 1 birth in 20 falls on the estimated date itself; most arrive in the fortnight either side.
- First babies tend to arrive a little later on average than subsequent ones.
- A dating scan in the first trimester overrides the period-based estimate when the two disagree by more than a few days.
Tips
- Use the first day your last period started, not the day it ended, or the date will read short.
- If you track your cycles, enter your real average length rather than leaving it at 28; it can move the date by several days.
- Treat the result as a window, not a deadline, and plan maternity leave and cover with a fortnight of slack on either side.
- Once you have had a dating scan, trust that date over this one and update any plans to match.
Frequently asked questions
Why is the count from my last period and not from conception?+
Most people know the date their last period started but not the exact day they conceived, so the 40-week clock starts from that first day by convention. It builds in roughly two weeks before conception even happens, which is why the figure can feel high at first.
How does cycle length change the date?+
Ovulation tends to sit about 14 days before the next period, so a longer cycle pushes conception later and the due date with it. The tool adds or subtracts a day for each day your cycle differs from 28.
How accurate is the estimate?+
It is a guide, not a fixed appointment. Only around 4 percent of babies arrive on the predicted day, and birth anywhere from 37 to 42 weeks is considered full term. An early dating scan is more precise and usually takes priority over the period-based date.
What if my cycle is irregular?+
Period-based dating assumes a fairly regular cycle, so an irregular one widens the margin of error. In that case the date from an ultrasound scan is the one to trust.
Does this work after IVF?+
For IVF the clinic dates the pregnancy from the egg collection or transfer day, which is more exact than a period date. Use those figures from your clinic rather than this estimate.
Things to watch
- This is general information, not medical advice. Book antenatal care with a midwife or doctor as early as you can.
- Period-based dating assumes a regular cycle and a 14-day luteal phase, so it is only an estimate and can be off by a week or more.
- A scan date from a clinician is more accurate and should always take precedence over this calculator.
Sources
- Due date calculator · NHS
- Methods for Estimating the Due Date (Committee Opinion 700) · American College of Obstetricians and Gynecologists
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with the official sources listed before making decisions.
Reviewed by Vikas Dulgunde.