Pregnancy Due Date Calculator
Calculate your estimated due date and see how many weeks pregnant you are.
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Pregnancy Information
How Due Dates Are Calculated
A pregnancy is conventionally counted as 40 weeks — 280 days — from the first day of your last menstrual period. That convention has an odd consequence that surprises many people: for the first two weeks of "pregnancy", you aren't pregnant at all. Dating from the last period is simply the most reliable date most people can recall, so medicine builds from it.
From your last menstrual period (Naegele's rule)
The standard method adds 280 days to the first day of your last period. The traditional shortcut, known as Naegele's rule, is to take that date, subtract three months, and add seven days and one year. If your last period began on 10 March, the estimated due date is 17 December.
This assumes a regular 28-day cycle with ovulation around day 14. If your cycles are consistently longer or shorter, the estimate shifts accordingly — someone with a 35-day cycle typically ovulates about a week later, pushing the true due date about a week later too.
From a known conception date
If you know when conception occurred — through fertility tracking, ovulation testing, or assisted reproduction — the calculation is 266 days from that date, since conception typically happens around two weeks after the period starts. Choose this method in the calculator if your conception date is more reliable than your period date.
How to Use This Calculator
- Choose a method: last menstrual period, or conception date.
- Enter the date: for the LMP method, use the first day of bleeding, not the last.
- Read the results: you'll get an estimated due date, how many weeks and days along you are today, and which trimester that falls in.
The Three Trimesters
| Trimester | Weeks | Roughly what's happening |
|---|---|---|
| First | 0–13 | Major organs and body systems form. Early symptoms such as nausea and fatigue are common. |
| Second | 14–27 | Growth accelerates; movement usually becomes noticeable. Often the most comfortable stretch. |
| Third | 28–40+ | Rapid weight gain and lung maturation, and the baby moves into position for birth. |
How Accurate Is a Due Date?
Less precise than the phrase "due date" suggests. Only around 4–5% of babies arrive on their estimated date. Most births happen within a window of roughly two weeks either side, and a full-term birth is anything from 37 to 42 weeks. It's more realistic to think of your due date as the midpoint of a likely range than as an appointment.
Several things affect accuracy:
- Cycle length and regularity. Naegele's rule assumes a textbook 28-day cycle. Irregular cycles make period-based dating less reliable.
- Uncertain LMP. Many people aren't sure of the exact first day, particularly if the pregnancy wasn't planned or if they were using hormonal contraception.
- Ultrasound dating. An early scan, ideally in the first trimester, measures the embryo directly and is generally the most accurate method available. Early-pregnancy growth is remarkably consistent between individuals, so measurements at that stage date a pregnancy well. If your scan date differs from your calculated date by more than a few days, your care team will usually adopt the scan date.
- First pregnancies tend, on average, to run slightly longer than subsequent ones.
Frequently Asked Questions
Why does the calculator say I'm two weeks pregnant when I only just conceived?
Because gestational age is counted from your last period, not from conception. It's a convention that keeps dating consistent across all pregnancies, since the date of a period is far more commonly known than the date of conception. Your "weeks pregnant" figure will always run about two weeks ahead of the actual age of the embryo.
My cycle isn't 28 days. Is this estimate wrong?
It will be somewhat off. As a rough adjustment, add the difference between your typical cycle length and 28 days. A consistent 32-day cycle suggests a due date about four days later than the standard calculation. An early ultrasound will settle it far more accurately than any arithmetic.
Does the due date change for twins?
The calculated date is the same, but multiple pregnancies very commonly deliver earlier, and they're monitored more closely as a result. Your care team will discuss what to expect.
Which date should I trust — this one or my scan?
Your scan, and more generally whatever date your healthcare provider has recorded. Ultrasound dating from the first trimester is the most reliable method available, and it's the one your maternity care will be based on. This calculator is a convenient estimate, not a substitute for clinical dating.
Disclaimer: This calculator provides a general estimate using standard dating conventions. It is not medical advice and cannot account for your individual circumstances. Always follow the dating and guidance given by your midwife, doctor or other healthcare provider.