Pregnancy Due Date Calculator
Estimate your due date, how many weeks along you are and your trimester, all calculated on your device
Calculation Details
How the estimated due date (EDD) works
A typical pregnancy timeline spans approximately 280 days (40 weeks) from the first day of the last menstrual period, assuming a standard 28-day cycle where ovulation happens around day 14.
Cycle length matters: if your cycle is usually longer or shorter than 28 days, ovulation shifts too, and the tool moves the due date to match.
Estimated Milestones
How to use this tool
- Pick the method: first day of your last period (LMP) or the date of conception.
- Enter that date. If you use LMP, set your usual cycle length with the slider (default 28 days; range 20–45).
- Read the estimated due date and the current gestational age, then copy the result if you want to share it with your care team.
- Remember it is an estimate: only a small share of babies arrive exactly on the due date.
Worked example
Suppose the last period started on August 27, 2026 and the cycle is 28 days. Naegele's rule adds 280 days (40 weeks) to that date, giving an estimated due date of June 3, 2027. With a 32-day cycle the tool shifts the date by the 4 extra days, to June 7, 2027, because ovulation is assumed to happen later. If an early ultrasound gives a different date, your clinician's dating takes precedence over this estimate.
Why this tool exists and where its numbers come from
Most people only know the first day of their last period, not the day of conception, so this tool turns that date into the estimated due date using the 280-day convention. Clinicians confirm or adjust the date with an early ultrasound; the tool does not replace that.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date (SMFM summary)
- ACOG Committee Opinion 579: Definition of Term Pregnancy (SMFM summary)
- StatPearls: Estimated Date of Delivery (280 days from LMP, Naegele's rule, redating thresholds)
For the full method, see the Methodology section just below.
Methodology
The Estimated Date of Delivery (EDD) is calculated with Naegele's Rule, adjusted for cycle length. Clinical convention counts the EDD as 280 days after the first day of the last menstrual period (LMP), assuming a regular 28-day cycle with ovulation around day 14, as described in the StatPearls chapter “Estimated Date of Delivery”. ACOG Committee Opinion 700 covers the methods for estimating the due date from the last period and an early ultrasound.
Where a due date drifts, and by how many days
Most of the drift in a due date comes from the input, not the arithmetic. Using March 10, 2026, as the first day of the last period (28-day cycle, result December 15, 2026):
- Entering the last day of bleeding. If the period ran March 10 to 14 and you type March 14, the tool returns December 19, four days late. Always use day one.
- Leaving the slider at 28 with a 35-day cycle. The tool adds (cycle − 28) days, so the right setting gives December 22. At 28 you would be a week early.
- Right date, wrong mode. Typing March 10 in conception mode adds 266 days instead of 280 and gives December 1, 14 days early.
- Trusting the slider with irregular cycles. It assumes ovulation always falls the same number of days before the next period. If your cycles vary, that shift is a guess.
- Treating the date as a deadline. Only a minority of babies arrive on it, and an early ultrasound may give a different date; your care provider's date wins. A future date or more than 300 days shows dashes for gestational age.
For general information only, not medical advice or official dating.
Helpful extras for your pregnancy
A pregnancy book, a body pillow and a planner: general items, not medical products
What to Expect When You're Expecting (Week-by-Week Guide)
A widely read week-by-week pregnancy book. It is general reading, not a substitute for the advice of your midwife or doctor.
Full Body Maternity Pregnancy Pillow
A full-length U-shaped body pillow, according to the listing, made to support side-sleeping.
Expectant Mother Pregnancy Planner & Journal
A pregnancy journal and appointment planner, according to the listing, for noting dates, questions for appointments and checklists.
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Frequently asked questions
Why is gestational age counted from the last menstrual period instead of conception? ▼
Because the exact date of conception is difficult to pin down without specialized monitoring, whereas the first day of the last menstrual period provides a clear calendar reference. This is why a pregnancy is called “40 weeks” even though conception happens roughly two weeks after that starting point, and why the conception mode here adds 266 days rather than 280.
How accurate is this estimated delivery date? ▼
Treat it as the center of a window, not a deadline. Only a minority of babies arrive on the exact calculated date, and pregnancies of 37 to 42 weeks are considered normal. The estimate is weakest with irregular cycles or an uncertain period date, because the formula assumes ovulation about two weeks before the next period would start.
How is the due date calculated after IVF? ▼
For pregnancies from IVF, ACOG Committee Opinion 700 dates the pregnancy from the embryo transfer, not from a period: the due date is 261 days after the transfer of a day-5 embryo (blastocyst) and 263 days after the transfer of a day-3 embryo. Choose the matching option and enter the transfer date; the tool then shows the same gestational age your clinic uses, counted as if from a last period. Your fertility clinic’s date takes precedence.
Why does my cycle length change the date? ▼
Naegele's rule assumes a 28-day cycle. If you ovulate later because your cycle is, say, 35 days, conception and therefore delivery happen about a week later than the rule predicts, which is why the slider adds or subtracts days from the result. Cycle length only helps if your cycles are fairly regular.
Can my doctor or midwife give me a different due date? ▼
Yes, and that date should win. An early ultrasound measuring the embryo from crown to rump is generally the most accurate way to date a pregnancy, and clinicians may revise the LMP-based date if the difference exceeds the thresholds above. This tool is for general information and does not replace medical advice.