Pregnancy Due Date Calculator
Estimate your due date, current week, and trimester from your last period.
This estimates a due date using Naegele's rule: the first day of your last menstrual period plus 280 days, which is 40 weeks. It is an estimate and should be treated as one — only about 4 percent of babies arrive on their estimated date, and roughly 80 percent are born within two weeks either side. An early ultrasound is a more accurate dating method and will often revise this figure. The rule assumes a regular 28-day cycle with ovulation on day 14, so a consistently longer or shorter cycle shifts the date by the difference. Because dating starts from the last period rather than conception, you are counted as roughly two weeks pregnant at the moment of conception, which is why gestational age runs ahead of embryonic age. A first-trimester scan measuring crown-rump length is accurate to within about five days and takes precedence clinically.
About the Pregnancy Due Date Calculator
Estimate your pregnancy due date using the standard Naegele's rule: 280 days from the first day of your last menstrual period (LMP). You'll also see how many weeks pregnant you are right now, which trimester you're in, and how many days remain until your estimated due date. This is an estimate only — always consult your healthcare provider for medical guidance.
How to use the Pregnancy Due Date Calculator
- 01Select the first day of your last menstrual period (LMP).
- 02Read your estimated due date.
- 03See your current pregnancy week, trimester, and days remaining.
Why use our Pregnancy Due Date Calculator
- 01
Naegele's rule
Due date = LMP + 280 days, the same method used by most providers.
- 02
Week & trimester
Automatically shows how far along you are today.
- 03
Countdown
Days remaining until your estimated due date.
- 04
Private
Your date is processed locally — nothing is uploaded.
How is a due date calculated?
Naegele's rule, formalised in the early nineteenth century, adds 280 days to the first day of the last menstrual period. Counting from the LMP rather than from conception is a convention rather than a biological statement: it means the first two weeks of a nominal pregnancy occur before conception has taken place, which is why gestational age is roughly two weeks ahead of the embryo's actual age. The rule assumes a regular 28-day cycle with ovulation on day 14. Anyone with a longer, shorter or irregular cycle will get a systematically shifted estimate — a 35-day cycle typically means ovulation around day 21, so the true due date is about a week later than the rule suggests. Some clinicians adjust for cycle length, and dating from a known conception date, which applies in IVF pregnancies, is more precise than dating from the LMP because the timing is documented rather than inferred.
Why ultrasound dating supersedes the LMP estimate
An ultrasound in the first trimester measures crown-rump length, and because early embryonic growth is remarkably consistent between pregnancies, that measurement predicts gestational age within about five to seven days. It is more reliable than an LMP-based estimate for most people, and considerably more reliable for anyone with irregular cycles or uncertain dates. Clinical guidance generally recommends revising the due date when the ultrasound estimate differs from the LMP estimate by more than about five to seven days in the first trimester. Accuracy declines as pregnancy progresses because growth variation between individuals widens — a third-trimester scan may be a week or more out, which is why dating is done early and then held rather than repeatedly revised. The practical consequence is that if your scan gives a different date from this calculator, the scan is the better number.
What the date is used for and why it matters clinically
The due date drives a great deal of the pregnancy care schedule: when screening tests are offered, when a pregnancy is considered term, and when induction is discussed. Full term is now defined as 39 weeks 0 days to 40 weeks 6 days, with 37 to 38 weeks 6 days classed as early term, 41 weeks as late term and 42 weeks and beyond as post-term — a refinement introduced because outcomes differ measurably across those bands. Accurate dating therefore affects real decisions, which is why it is worth resolving discrepancies early rather than carrying two competing dates. Being past the estimated date is common and not in itself a problem; most guidelines discuss induction somewhere between 41 and 42 weeks. This calculator is a convenience for planning and for satisfying curiosity, not a clinical tool — antenatal care, and any concern about dates or symptoms, belongs with a midwife or doctor.
Gestational age terminology
| Term | Weeks | Note |
|---|---|---|
| Preterm | Before 37 weeks | Further subdivided by severity |
| Early term | 37w0d – 38w6d | Outcomes differ from full term |
| Full term | 39w0d – 40w6d | Optimal window |
| Late term | 41w0d – 41w6d | Induction usually discussed |
| Post-term | 42w0d and beyond | Increased monitoring |
About 4% of babies arrive on the estimated date; roughly 80% arrive within two weeks either side.
Frequently asked questions
How is the due date calculated?
Naegele's rule adds 280 days — 40 weeks — to the first day of your last menstrual period, assuming a regular 28-day cycle with ovulation on day 14.
How accurate is it?
It is an estimate. Only about 4% of babies are born on the date, and roughly 80% arrive within two weeks either side of it.
Why does it count from my last period rather than conception?
It is a dating convention. It means the first two weeks of a nominal pregnancy precede conception, so gestational age runs about two weeks ahead of the embryo's actual age.
What if my cycle is not 28 days?
The estimate shifts. A 35-day cycle usually means ovulation around day 21, so the true date is roughly a week later than the rule suggests.
Is an ultrasound more accurate?
Yes, in the first trimester. Crown-rump length predicts gestational age within about five to seven days, and clinicians usually revise the date if it differs by more than that.
Why is a late scan less accurate for dating?
Growth variation between individuals widens as pregnancy progresses, so a third-trimester measurement can be a week or more out. Dating is done early and then held.
What counts as full term?
39 weeks 0 days to 40 weeks 6 days. 37 to 38w6d is early term, 41 weeks is late term and 42 weeks or more is post-term.
Is my date of birth or period data stored?
No. The calculation happens entirely in your browser and nothing you enter is transmitted or saved.
Is this a substitute for antenatal care?
No. It is a planning convenience. All antenatal care and any concern about dates or symptoms should go to a midwife or doctor.
Last updated
Naegele's rule as used in standard obstetric practice. Term definitions per ACOG and the WHO. Ultrasound dating accuracy per NICE antenatal care guidance NG201. Not medical advice.