The short answer
Use the due date calculator, enter the first day of your last menstrual period, and you get an estimated due date — but the right way to use that date is as a starting point, not a conclusion. The logic has three steps. First, the default method is Naegele's rule: the first day of the last menstrual period plus 280 days (40 weeks). Second, if your cycle is not a tidy 28 days, or conception did not happen naturally, that date drifts — the direction and size of the drift follow your cycle length. Third, the early pregnancy ultrasound (the one done at 6–12 weeks) re-derives the due date from the actual size of the embryo, and from that point on the ultrasound weeks are the ones your doctor uses, and the calculated date should be discarded. So: the date calculated from your last period is a draft; the ultrasound weeks are the final version. If you are still trying to conceive or have just seen a positive test and have not registered for care yet, calculating a rough date is genuinely useful for booking appointments, arranging leave, hiring a maternity nurse and buying baby things. Just do not compare that number with anyone else's, and do not read it as a prediction of early or late arrival.
Where the 280-day number comes from
280 days is not arbitrary: it is the real length of an average pregnancy. The average interval from ovulation to delivery is about 266 days, and ovulation happens roughly 14 days into a cycle, giving 280 days in total. In weeks that is 40, so the two formulas are mirrors of each other: gestational age = (today − first day of last period) ÷ 7, and due date = first day of last period + 280 days. Three details get skipped constantly. First, gestational age is quoted in weeks and days, not whole weeks — 37 weeks and 3 days is genuinely four days short of 38, and rounding to whole weeks makes people think the due date has arrived. Second, the first day of the last period means the first day of bleeding, not the last; people usually remember "roughly when it came", and being three or five days out is common, which shifts the whole due date. Third, 28 days is an average, not a standard: a 21-day or a 35-day cycle can both be perfectly regular. So the accurate statement of Naegele's rule is that it is most reliable for people with a regular cycle of about 28 days. A 25-day cycle yields a due date that runs late, a 32-day cycle yields one that runs early, and the offset is roughly the number of days your cycle differs from 28.
The cases where the calculated date is clearly off
Four situations make the last-period method unreliable, and each deserves separate mention. First, an irregular cycle — if your cycle floats anywhere between 21 and 35 days, or is often delayed by a week or two, the error can reach several days or even a fortnight. The method works well for regular cycles and should not be over-trusted for irregular ones. Second, conceiving again before ovulation has resumed after breastfeeding: cycles before ovulation returns tend to run long, so the calculated due date comes out noticeably late, and some clinicians will go straight to the ultrasound weeks. Third, assisted reproduction (IVF): the transfer date is known exactly, so weeks should be counted from the transfer rather than from the last period (some clinics align using "last period + 14 days" — follow your own doctor). Fourth, conditions such as PCOS that make ovulation irregular: the conception date itself is hard to pin down, and the error from a last-period calculation can be two or three weeks. The operable summary is: trust the first estimate only if your cycle is regular, around 28 days, and you are clear about the first day. Otherwise wait for the ultrasound to give you a more reliable version.
How ultrasound corrects the due date, and why it is better
The early pregnancy ultrasound (usually the 6–8 week scan, transvaginal or abdominal) measures the actual crown-rump length of the embryo and back-calculates gestational age against a standard growth curve. It beats the last-period method because it measures the fetus directly instead of relying on indirect information like "when do you remember the period starting" or "how regular is your cycle". Your clinician compares the ultrasound weeks with your own calculation: if they differ by under 5 days, the last-period date usually stands; if they differ by more than 5 days, the ultrasound wins, and the corrected date then governs the whole pregnancy. That makes one thing worth doing actively at registration: tell the clinic the exact date of the first day of your last period, even if you have already run it through a calculator. After the correction, every appointment in the schedule — the NT scan, the screening test, the anomaly scan, the non-stress test — is timed against the corrected weeks, so do not re-derive the timeline yourself from the last period; that shifts the entire rhythm out of step. If you want the timeline laid out clearly, use a date calculator to check which week each milestone falls into.
What 37 weeks, 40 weeks and 41 weeks actually mean
A due date is a point, but clinically what matters is a range. The usual breakdown: 37 weeks 0 days to 39 weeks 6 days counts as term (the organs are essentially mature and the risk of prematurity drops sharply); 40 weeks exactly is the due date itself; and anything past 41 weeks (some standards say 41+6) counts as post-term, where placental function declines and amniotic fluid volume drops, so clinicians usually suggest induction around 41 weeks rather than waiting indefinitely. So "still not here after the due date" is not itself a red flag — only about 5% arrive on the day, and intervention is what matters past 41 weeks. The same logic runs the other way: being born before the due date is not a problem either, since anything from 37 to 42 weeks is within the normal range. Genuine neonatal concern starts below 37 weeks. This range has a practical side effect: the closer you get to the due date, the earlier preparation should be. Packing the hospital bag before 36 weeks and sorting out the route and the paperwork matters more than staring at that one date.
Putting the due date to use: three things worth doing
After you have a date, the useful response is not to keep re-checking it but to do three things. One, turn it into a timeline: counting back from the due date lays out the approximate milestones — early pregnancy ultrasound at 6–8 weeks, NT at 11–13⁺⁶ weeks, screening at 15–20 weeks, the anomaly scan at 20–24 weeks, the glucose test at 24–28 weeks, appointments every two weeks from 28 weeks and every week from 36 weeks. With that line drawn, appointments do not get missed. Two, set a reminder — but not a countdown to zero that implies an outcome: use a countdown tool for the due date so family knows roughly which window you are in and can plan leave, rather than manufacturing anxiety. Three, work out a graded rule for when to go to hospital: bleeding, irregular contractions and water breaking differ in urgency. If the waters break, lie down and contact the hospital promptly whether or not contractions have started (especially if the head is not engaged); for regular contractions use the 5-1-1 rule (every 5 minutes, lasting 1 minute, for 1 hour); for bleeding, go when contractions become regular. These criteria decide how long you wait outside the delivery room far more than the due date does.
想马上动手：用 预产期计算器 算初算预产期与当前孕周，算日子差用 日期计算器，设提醒用 倒计时，备产时想知道孕周对应哪些检查节点用 年龄与日期推算。Try it now: estimate the due date and current gestational age with the due date calculator, check intervals with the date calculator, set a reminder with the countdown tool, and work out week-to-week milestones with date and age math.
FAQ
How accurate is a due date? Mine never matches.
A due date is an estimate, not a guarantee. Only around 5% of babies arrive on the day itself, and births between 37 and 42 weeks all count as normal, so a mismatch usually means a statistical midpoint was treated as a fixed appointment. The usual causes are a misremembered first day (by far the most common), a cycle that is not 28 days, or ordinary natural variation. The real fix is the early ultrasound: measuring crown-rump length and back-calculating gives weeks with under a week of error, more reliable than counting from the last period. At registration, give the clinic the first day of your last period yourself. If it differs from the ultrasound weeks by more than 5 days, the hospital corrects the due date, and every appointment afterwards is scheduled against that corrected date.
I cannot remember the first day of my last period. Can I still calculate?
You can, but only roughly, and you should not treat it as precise. A workable approach: work out roughly which week the period fell in, take a middle day of that week as an approximation, and use it to get a rough range; then rely on the early pregnancy ultrasound. At 6 to 8 weeks, the crown-rump length gives weeks that beat any recollection. When the clinic asks, give them the range you are confident of (say, "somewhere between 5 and 12 March") rather than a made-up specific date, and let them combine it with the scan. Also, do not keep editing the calculated date because it looks wrong — repeated edits only add anxiety, and the due date is the clinician's to adjust, not the calculator's.
I am past my due date — should I ask about inducing labour?
It is not for you to judge, and there is no need to panic. A week past the due date but still inside 41 weeks falls within routine follow-up; most people go into labour in that window, and appointments are normally weekly by then with the clinician monitoring the non-stress test, ultrasound, placental function and amniotic fluid. The point at which induction actually gets discussed is past 41 weeks (some standards use 41 weeks 6 days), where the risk of declining placental function and reduced amniotic fluid rises, and the clinician may recommend hospital assessment and induction. That decision belongs to the obstetric team and depends on placental status, fetal size, fluid index and your own circumstances — not on a countdown on your phone. If you want the process clear in your head, use a countdown tool to mark the key thresholds after 37 weeks (term, 40 weeks, 41 weeks assessment) so you understand how the sequence runs rather than checking every morning.