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Pregnancy Due Date Calculator

Enter the first day of your last period for an estimated due date, gestational age and trimester timeline, with an optional adjustment for your average cycle length. It is an estimate, not a confirmed date.

Your last period

Enter the first day your most recent period began, not the day it ended.
days
Leave this at 28 days if you are unsure. Shorter or longer cycles shift the estimate earlier or later.

This calculator uses a standard 280-day (40-week) pregnancy estimate and assumes a regular 28-day cycle unless you choose a different average cycle length.

Your estimated due date
Enter the first day of your last period to see your estimated due date

How to use this calculator

  1. Enter the first day of your last period in the date field — the day it began, not the day it ended. A date in the future is not accepted, and a date more than a year ago shows an informational message instead of a due date.
  2. If your cycles are usually shorter or longer than 28 days, set your average cycle length — a whole number from 15 to 60 days. Leave it at 28 if you are unsure.
  3. Your estimated due date appears straight away, with the figures picked out in orange, along with your estimated gestational age and the estimated time until the due date.
  4. Expand How the estimate is calculated to see the formula and a worked example. The timeline shows the approximate start of each trimester and the estimated due date. Use Reset to clear the date and restore the 28-day cycle.

This calculator uses a standard 280-day (40-week) pregnancy estimate. It assumes a regular 28-day cycle unless you choose a different average cycle length. The result is an estimate only — not a diagnosis, confirmation of pregnancy or clinical dating — and your midwife or maternity team may confirm or revise it after an ultrasound dating scan.

How is the due date worked out?

A full-term pregnancy is estimated at 280 days, or 40 weeks, measured from the first day of your last menstrual period (LMP). The calculator then shifts that estimate by the difference between your average cycle length and 28 days:

EDD = LMP + 280 days + (cycle length − 28)
  • EDD — estimated due date
  • LMP — first day of your last menstrual period
  • cycle length − 28 — the cycle adjustment: 0 for a 28-day cycle, +2 for a 30-day cycle, −2 for a 26-day cycle

Example: a last period starting 1 January 2026 with a 28-day cycle gives an adjustment of 0. Adding 280 days lands on 8 October 2026. With a 30-day average cycle the adjustment is +2, moving the estimate to 10 October 2026.

The cycle-length adjustment applies to the estimated due date only. Estimated gestational age and the trimester milestones are counted from the first day of your last period and are not shifted by your cycle length. This calculator estimates gestational age from the first day of the last period; it does not identify the exact date of ovulation or conception.

When does each trimester start?

The trimester milestones are approximate reference points counted from the first day of your last period. They are not confirmed clinical dates or an antenatal-care schedule.

Approximate trimester milestones
MilestoneTimingDays from LMP
Approximate end of first trimester12 weeks and 6 days90
Approximate start of second trimester13 weeks91
Approximate start of third trimester28 weeks196
Estimated due date40 weeks280 + (cycle length − 28)

Only the estimated due date changes with your cycle length — a longer cycle moves it later, a shorter cycle earlier. The trimester milestones stay anchored to your last period date.

How accurate is the estimated due date?

This is an estimate only. Pregnancy length varies, and dates calculated from your last menstrual period may not be exact, especially if your periods are irregular or you are unsure of the date. Your midwife, GP or a dating scan can provide a more accurate estimated due date.

A dating scan, usually offered between 8 and 14 weeks, can estimate how far along the pregnancy is and may adjust your estimated due date. Due dates are estimates, so babies can arrive before or after this date. The method here follows the NHS due date estimate of 280 days from the first day of the last period, adjusted for the chosen average cycle length. The NHS week-by-week pregnancy guide explains what happens at each stage.

If you have pain, bleeding or concerns about your pregnancy, contact your midwife, GP, maternity unit or NHS 111. In an emergency, call 999.

What this calculator does not provide

  • Pregnancy confirmation or a positive test result
  • Confirmation that a pregnancy is ongoing or healthy
  • A medical diagnosis or risk assessment
  • The date of ovulation or conception
  • An IVF or embryo-transfer due-date calculation
  • Clinical dating, scan dates or an antenatal-care schedule
  • A substitute for a dating scan, midwife, GP, maternity unit or other healthcare professional

Frequently asked questions

How is the estimated due date calculated?

The calculator adds 280 days — 40 weeks — to the first day of your last menstrual period (LMP), then adjusts by your average cycle length minus 28 days. A 30-day cycle pushes the estimate two days later; a 26-day cycle brings it two days earlier. The full expression is: estimated due date = LMP + 280 days + (cycle length − 28). Dates use real calendar arithmetic, so leap years and different month lengths are handled correctly.

What is estimated gestational age?

Gestational age is how far along a pregnancy is, counted from the first day of the last menstrual period — not from conception. The calculator subtracts that date from today, divides by 7 for completed weeks, and shows the leftover days, for example "approximately 12 weeks and 3 days". Because it is based on the last period rather than a scan, it is an estimate. It is not fetal age or a conception age. Gestational age is not adjusted for cycle length; only the due date is.

Why does the calculator ask for my average cycle length?

The 280-day estimate assumes a regular 28-day cycle. If your cycles are consistently shorter or longer, the estimated due date shifts by the difference between your cycle length and 28 days — a 30-day cycle moves it two days later, a 26-day cycle two days earlier. Leave the field at 28 if you are unsure. The calculator accepts whole numbers from 15 to 60 days; 21 to 35 days is treated as the typical range, and a value outside it is flagged as less reliable. Cycle length changes the due date only, not gestational age or the trimester milestones.

Why might a dating scan give a different due date than this estimate?

This estimate uses only the dates you enter and assumes a regular 28-day cycle. A dating scan, usually offered between 8 and 14 weeks, measures the baby directly and does not depend on your cycle or your memory of your last period, so it is more accurate — especially if your periods are irregular or the date is uncertain. Your midwife or maternity team may confirm or revise your due date after that scan. Only about 1 in 20 babies is born on the estimated due date; most arrive in the two weeks either side.

What if my cycles are irregular or I am not sure of my last period date?

The estimate is least reliable in exactly those situations, because it assumes regular cycles and a known first day of the last period. If your cycles vary from month to month, fall outside the typical 21-to-35-day range, or you are unsure of the date, treat the result as a rough guide. An ultrasound dating scan does not rely on your cycle and gives a more reliable estimated due date. Speak to a midwife, GP or maternity team for advice specific to you.

Does this calculator tell me the date of conception?

No. This calculator estimates dates from the first day of the last period and average cycle length. It does not identify the exact date of ovulation or conception.

Is this result confirmation of pregnancy?

No. It is a date estimate worked out from the dates you enter. It does not test for, diagnose or confirm a pregnancy, and it cannot tell you whether a pregnancy is ongoing or healthy. Only a pregnancy test and your midwife, GP or maternity team can confirm a pregnancy and its dating.

What does this calculator not do?

It does not confirm a pregnancy, provide a medical diagnosis or risk assessment, identify the date of ovulation or conception, or calculate a due date from IVF or embryo transfer. It is not a substitute for a dating scan or for advice from a midwife, GP or maternity unit.

Are my dates sent anywhere?

No. The calculation runs entirely in your browser. Nothing is submitted, stored or logged, and the page makes no network request when you enter a date. Your last-period date and cycle length are not saved.