What is a Due Date Calculator?
A due date calculator estimates the day a pregnancy is expected to reach forty weeks, which is how obstetric practice defines term. The standard method is Naegele's rule: take the first day of the last menstrual period, add 280 days. Everything else in modern practice is a variation on the same arithmetic, differing only in which event is counted from — a known ovulation or conception date, or an IVF transfer whose embryo age is known to the day.
The convention of counting from the last period rather than from conception is deliberate and worth understanding, because it explains why gestational age looks two weeks too high. On an average twenty-eight-day cycle, ovulation happens about fourteen days after the period begins, so by the time conception occurs the pregnancy is already being counted as two weeks old. A person who is four weeks pregnant by this reckoning has missed a period by days, not by a month.
Accuracy depends on when you know what. Counting from a last period assumes a regular cycle and ovulation on day fourteen, so a thirty-two-day cycle shifts the estimate by four days and an irregular cycle by an unknown amount. A known ovulation date, a recorded conception, or an IVF transfer removes that assumption entirely. The most accurate dating, however, is an early ultrasound that measures crown-rump length — which is why most services treat the scan as the dating reference and this arithmetic as the starting estimate.
How to estimate a due date
- Choose what you are counting from: the first day of the last period, a known conception or ovulation date, or an IVF transfer with its embryo stage.
- Enter that date. If you are counting from a last period, also give your average cycle length so ovulation can be placed correctly rather than assumed at day fourteen.
- Read the estimated due date, and the implied last period beneath it — gestational age is always counted from that date, even when the anchor was something else.
- Look down the milestone table for the trimesters, the anatomy scan window, the viability threshold and the early-term point.
- Compare the estimate against any early dating scan you have. If the two differ by more than about a week, the scan is the reference that matters.
- Treat the date as the centre of a five-week window, not as an appointment — only about four per cent of babies arrive on it.
Due Date Calculator limits and rules
| Item | Limit |
|---|---|
| Rule used | Naegele's rule: 280 days from the first day of the last menstrual period |
| Anchors offered | Last period, conception or ovulation, IVF day-5 blastocyst, IVF day-3 embryo |
| Offsets | 280 days, 266 days, 261 days and 263 days respectively |
| Cycle length | 20 to 45 days, applied only when counting from a last period |
| Accuracy of the last-period method | Assumes ovulation on day 14; a 32-day cycle moves the estimate by four days |
| Term window | 37 to 42 weeks, which is a five-week span |
| Chance of arriving on the date | Around 4 per cent |
| Not a substitute for | An early dating ultrasound, which measures the embryo rather than counting days |
When to use it
- Getting a first estimate before the dating scan, so appointments and work plans can be sketched out.
- Working out dates after a frozen embryo transfer, where the embryo's age is known precisely and the arithmetic is therefore exact.
- Reconciling a due date a midwife gave with the dates you have written down.
- Understanding which week of pregnancy you are in when a test or scan is being scheduled.
- Checking that a conception date and a last-period date actually agree, when they seem to be several days apart.
Is it private?
The dates you enter never leave your browser. The calculation is date arithmetic in JavaScript — adding 280 days to a timestamp — and there is no request, no storage and no account. Nothing about this page is retained after you close it, which matters for information this personal.
Frequently asked questions
It is a reasonable starting estimate and a poor final one. The method assumes a twenty-eight-day cycle with ovulation on day fourteen, which is true for a minority of people; a thirty-two-day cycle moves the estimate four days later, and irregular cycles make it unreliable altogether. It also depends on remembering the date correctly. An early ultrasound measuring crown-rump length is more accurate, with a margin of about five to seven days in the first trimester, and it is why most services redate the pregnancy after the scan.
It is a convention that predates ultrasound and has stuck because it is the one date every pregnant person can identify without knowing exactly when conception happened. The cost is a two-week offset: by the convention, you are already four weeks pregnant on the day of the missed period, even though conception occurred roughly a fortnight earlier. It matters when interpreting test windows and scan timing, which is why the calculator shows both the implied last period and the due date.
By counting from the transfer rather than from a period, which removes all of the uncertainty: the embryo's age is known exactly. For a day-5 blastocyst, add 261 days; for a day-3 embryo, add 263. This gives the same forty-week mark as the conventional method, without the assumption about cycle length. It is the most precise version of the calculation available without a scan.
That a birth anywhere from 37 to 42 weeks is considered term, which is a spread of five weeks. Thirty-seven to thirty-nine weeks is called early term, thirty-nine to forty-one full term, and beyond forty-one late term. Induction is usually discussed when a pregnancy passes forty-one or forty-two weeks, because the risk of stillbirth and other complications rises gradually beyond that point. This is why the due date is best read as the middle of a window rather than as a deadline.
Yes, and it frequently is. If an early ultrasound disagrees with the date from the last period by more than about seven days in the first trimester, the scan date usually takes precedence, because ultrasound measures the embryo directly. Later scans are less accurate for dating because growth rates vary more as the pregnancy progresses, which is why a scan done at thirty weeks never changes the dates.
No. It is arithmetic on dates you supply, and it cannot know anything about your pregnancy. It does not detect problems, does not replace antenatal care, and does not account for the many medical reasons individual pregnancies are managed to a different schedule. Any question about induction, monitoring, testing or symptoms belongs with your midwife or obstetrician, who will be working from your scan results.
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Written and maintained by Md. Nur Hossain Repon.