Pregnancy Due Date Calculator
Estimate your baby's due date from the first day of your last period. See how many weeks along you are, your current trimester, a countdown to your due date, and your estimated conception date.
What an estimated due date means
The estimated due date (EDD) is the day around which your baby is most likely to arrive — 40 weeks from the start of your last period. It's a helpful planning anchor for prenatal appointments, tests and time off work, but it's genuinely an estimate, not a deadline. Only about 1 in 20 babies (around 4–5%) arrive on the exact due date; the great majority are born in the two weeks before or after. Thinking of it as a "due window" rather than a single day tends to be far less stressful as the date approaches.
This calculator estimates your EDD from the first day of your last period, adjusts for your cycle length if you provide it, and shows how many weeks along you are, your current trimester, and an estimated conception date.
How to use this calculator
Enter the first day of your last menstrual period (LMP) — the first day of bleeding, not when it ended — and, if you know it, your average cycle length. The default assumes a 28-day cycle; if yours is longer or shorter, providing it lets the tool adjust for when you likely ovulated. It then returns your estimated due date along with how far along you are today.
How it's calculated
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last period. A common shorthand is to take that date, subtract three months and add seven days and a year. The rule assumes a regular 28-day cycle with ovulation around day 14, so if your cycle is longer or shorter (the calculator accepts 20 to 45 days) it shifts the due date by the difference — and shifts the weeks-along count and estimated conception date by the same number of days, so the figures stay consistent. Note that this counts pregnancy from your last period — about two weeks before conception actually happens — which is why "weeks pregnant" is always roughly two weeks ahead of the embryo's true age.
When cycles are irregular or the LMP is uncertain, dating from an early ultrasound scan is more accurate. A first-trimester scan measures the baby's size to estimate age and is considered the most reliable way to set or confirm a due date; if it differs significantly from the LMP estimate, providers usually go with the scan.
Worked example
If the first day of your last period was 1 March, Naegele's rule adds 280 days to give an estimated due date of about 6 December the same year. By the rule-of-thumb shortcut: 1 March − 3 months = 1 December, + 7 days = 8 December (the small difference reflects month lengths). Either way, the actual birth could reasonably land any time from late November to mid-December.
| Stage | Weeks + days | Milestones |
|---|---|---|
| First trimester | Up to 13w 6d | Major organs form; dating scan; higher miscarriage risk early on |
| Second trimester | 14w 0d – 27w 6d | Anatomy scan (~18–22 wks); movements felt; "viability" approached near the end |
| Third trimester | 28w 0d onward | Rapid growth; baby settles head-down; closer monitoring |
| Early term | 37w 0d – 38w 6d | Term, though breathing problems are more common than at full term |
| Full term | 39w 0d – 40w 6d | The due date (40w 0d) falls in this window |
| Late term | 41w 0d – 41w 6d | Past the due date — follow your provider's plan |
| Post-term | 42w 0d onward | Beyond 42 weeks — follow your provider's plan |
Limitations & tips
Births from 37 weeks onward count as term: ACOG calls 37 weeks 0 days to 38 weeks 6 days early term, 39 weeks 0 days to 40 weeks 6 days full term and 41 weeks 0 days to 41 weeks 6 days late term, so a birth a week or two either side of the due date is common. This LMP-based method is least reliable when periods are irregular, when the last-period date is unknown, or after recent hormonal contraception — in those cases an ultrasound dating scan is the better source. Most importantly, this tool gives general information only and is not medical advice or a diagnosis. It cannot replace prenatal care: please confirm your due date and look after your pregnancy with a midwife, obstetrician or other qualified healthcare provider, and contact them promptly about any concerns or warning signs.
Frequently asked questions
How is my due date calculated?
By Naegele's rule: 280 days (40 weeks) are added to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the calculator adjusts the date for when you likely ovulated.
How accurate is a due date?
It's an estimate. Only around 4–5% of babies are born on their exact due date; most arrive within two weeks either side. An early ultrasound dating scan is the most accurate way to date a pregnancy.
What if I don't know my last period date?
The last-period method needs that date. If you're unsure, or your cycles are irregular, an ultrasound dating scan from your healthcare provider is the best way to estimate your due date.
Why does the due date count from my last period, not conception?
Most people know the date their last period started, but rarely the exact day of conception, which usually happens around two weeks later. Dating from the last period gives a consistent reference, which is why "weeks pregnant" runs about two weeks ahead of the embryo's actual age.
What counts as full term?
The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days, with the due date at 40 weeks. Births at 37–38 weeks are early term, at 41 weeks late term, and from 42 weeks post-term. If your pregnancy goes past the due date, your provider will talk you through the plan.
Is this calculator a substitute for seeing a doctor?
No. It offers a general estimate for planning only and is not medical advice. Confirming your due date and caring for your pregnancy should always be done with a midwife, obstetrician or other qualified provider, who can use ultrasound and examinations for accurate dating.
Sources & references
- American College of Obstetricians and Gynecologists — Committee Opinion No. 700, "Methods for Estimating the Due Date" — official guidance on dating pregnancies and when ultrasound should take precedence over LMP-based estimates.
- American College of Obstetricians and Gynecologists — Committee Opinion No. 579, "Definition of Term Pregnancy" — source of the early-term (37w 0d–38w 6d), full-term (39w 0d–40w 6d), late-term (41w 0d–41w 6d) and post-term (42w 0d onward) definitions.
- ACOG — How Your Fetus Grows During Pregnancy — trimester definitions: first trimester to 13 weeks 6 days, second from 14 weeks 0 days to 27 weeks 6 days, third from 28 weeks 0 days.
- "Naegele's rule and the length of pregnancy – a review" — history and accuracy of the 280-day (40-week) rule this calculator uses.
Last updated: September 2026 ·How we calculate
BriskToolbox provides estimates for general information only and is not medical advice. Talk to a healthcare professional about your individual health.