About the Pregnancy Due Date Calculator
A free tool that estimates your due date from three standard methods — last period, conception date, or ultrasound — and shows your current gestational age, trimester, and normal arrival window.
What it does
The Pregnancy Due Date Calculator estimates when your baby is likely to arrive based on one of three clinical dating methods. Enter the first day of your last menstrual period and the tool applies Naegele's rule (LMP + 280 days) to produce an estimated due date (EDD). You can also adjust for a non-28-day cycle, switch to a conception or ovulation date, or enter your ultrasound scan date with the gestational age your sonographer measured — whichever gives the best-known starting point.
Beyond the date itself, the tool shows how far along you are right now in weeks and days, which trimester you're in, and a normal due window (37–42 weeks) to make clear that the EDD is an estimate, not a deadline. Everything runs in your browser — no data is stored or sent anywhere.
Key features
Three dating methods
Last menstrual period (with optional cycle-length adjustment), conception or ovulation date, and ultrasound dating using the gestational age at scan. Choose the method that matches what you know.
Estimated due date (EDD)
The EDD is displayed prominently, along with a days-to-go countdown and a plain note about how far away it is — or how many days past the EDD if you are in the late stages.
Current gestational age
Shows your current gestational age in weeks and days, calculated from the effective LMP date to today, updating automatically on each use.
Trimester indicator
First trimester covers weeks 1–13, second trimester weeks 14–27, and third trimester week 28 onward. A colour-coded badge makes it easy to see at a glance.
Normal due window
Shows the 37–42 week date range around your EDD — the normal range of term delivery — so the EDD is framed as the midpoint of a window, not a fixed deadline.
Pregnancy progress timeline
A visual progress bar spanning weeks 0–40 shows where you are in the pregnancy at a glance. It adapts to your current gestational age each time you open the tool.
The dating methods
Last menstrual period (Naegele's rule)
EDD = LMP + 280 days. The most common clinical method, assuming a 28-day cycle with ovulation at day 14. For other cycle lengths the tool adds (cycle length − 28) days to adjust the ovulation assumption. Accurate when the LMP date is reliably known.
Conception / ovulation date
EDD = conception date + 266 days (38 weeks). Useful when the date of conception or ovulation is known from tracking, assisted reproduction, or a positive ovulation test. Equivalent to LMP dating but anchored to fertilisation rather than the last period.
Ultrasound dating
The EDD is back-calculated from the scan date and the gestational age measured by the sonographer. A first-trimester scan (before 13–14 weeks) is the most accurate method clinically, and per ACOG guidelines takes precedence over LMP dating when the two differ by more than about 7 days.
Cycle-length adjustment
When using the LMP method, entering a cycle length other than 28 days shifts the EDD proportionally. This reflects that ovulation — and therefore fertilisation — typically occurs around day 14 of the cycle, so a longer cycle delays it and a shorter cycle advances it.
Why the EDD is an estimate, not a deadline
Ovulation varies
The LMP method assumes ovulation on day 14, but actual ovulation can shift by several days in either direction, even in regular cycles. This alone can move the true due date by a week or more.
Implantation timing differs
After fertilisation, implantation typically occurs 6–10 days later, but the range varies between individuals, adding further uncertainty to the calculated EDD.
Fetal growth is individual
Ultrasound-based gestational age estimates have a margin of roughly ±1–2 weeks in the first trimester and wider later on, because fetal size is naturally variable between pregnancies.
Normal range is wide
Births between 37 and 42 weeks are considered normal term deliveries. Planning around the EDD is useful; treating it as a firm date is not — and clinicians have known this for decades.
This tool provides an estimated due date only — only about 5% of babies arrive on the exact EDD, and a normal arrival window spans weeks 37–42. A first-trimester ultrasound is generally more accurate than period-based dating and takes precedence when the two differ by more than about a week (per ACOG guidelines). This estimate is not medical advice. Please begin prenatal care early and confirm your dating with your doctor or midwife — they can give you the most accurate information and personalised care throughout your pregnancy.
Estimate your pregnancy due date
Free, no sign-up required. Works on any device.
Open the Due Date Calculator