How a Due Date Is Worked Out
A due date is an estimate, and the standard way of producing it is older and simpler than most people expect. Naegele’s rule takes the first day of your last menstrual period, adds one year, subtracts three months and adds seven days, which comes to 280 days, or forty weeks. It assumes a 28-day cycle with ovulation on day 14.
This calculator applies that rule and lays out the weeks that follow, so you can see where any given date falls in the pregnancy rather than just when it is expected to end.
How to Use the Due Date Calculator
One date drives everything, and which date you use matters.
- Enter the first day of your last menstrual period, the day bleeding began, not the day it ended.
- If your cycle is not 28 days, note that the estimate will be off by roughly the difference. A regular 32-day cycle usually means ovulation around day 18, pushing the estimate about four days later.
- Read the estimated due date and the current gestational age in weeks and days.
- Use the milestone table below to see where you are relative to the trimesters and the usual points of care.
- If you have had a dating ultrasound, use the date it gave. A first-trimester scan is more accurate than Naegele’s rule and is what your clinician will work from.
Why Estimates Get Revised
Naegele’s rule is a population average applied to an individual, and it rests on an assumption, the 28-day cycle, that only a minority of people actually have. If your cycles run long, short or irregular, the estimate inherits that error directly, which is why a date from a scan usually supersedes it.
An ultrasound in the first trimester measures the embryo directly, and at that stage growth is consistent enough between pregnancies that it gives a date accurate to within about five days. Later scans are less useful for dating because growth rates diverge, which is why the dating scan is done early and the date is not usually revised again afterwards.
Pregnancy Timeline by Week
Gestational weeks are counted from the first day of the last period, so weeks one and two precede conception. This is the convention clinicians use.
| Weeks | Stage | What typically happens |
|---|---|---|
| 1–4 | First trimester | Counting begins; conception around week 2–3 |
| 5–8 | First trimester | Pregnancy usually confirmed; first appointment often booked |
| 9–13 | First trimester | Dating scan commonly offered around 11–13 weeks |
| 14–27 | Second trimester | Anatomy scan around 18–22 weeks |
| 28–36 | Third trimester | More frequent appointments; monitoring increases |
| 37–38 | Early term | Considered term from 37 weeks |
| 39–40 | Full term | The estimated due date falls at 40 weeks |
| 41–42 | Late term | Post-term from 42 weeks; management varies by provider |
Only about four per cent of babies arrive on the estimated date itself, and the large majority arrive within two weeks either side of it. The date is a midpoint, not a deadline.
This Is an Estimate, Not Medical Guidance
The calculator applies a standard formula to one date. It knows nothing about your cycle length, your medical history, whether the pregnancy is a multiple, or anything a clinician would take into account. Where it disagrees with a date you have been given by a midwife or doctor, theirs is the one to use.
This page is general information and not medical advice. Anything that involves a decision, appointments, tests, symptoms that concern you, or any bleeding or pain, belongs with your healthcare provider rather than with a formula, and urgent symptoms warrant contacting them straight away.