Calculating your due date
How is a due date calculated?
By convention, the estimated due date is 280 days after the first day of the last menstrual period.1 For IVF pregnancies it is based on the transfer date instead: 263 days after a day-3 embryo transfer or 261 days after a day-5 transfer.1 Read more: How a due date is calculated.
Can I calculate my due date from my last period?
Yes — that’s the standard method. Add 280 days to the first day of your last period.1 For example, a last period starting on 9 July 2026 gives an estimated due date of 15 April 2027. You can calculate your estimated due date here.
Can I calculate my due date from my conception date?
Yes, by convention: add 266 days to an estimated conception date. That interval is 280 days minus the 14 days the convention assumes before ovulation.1
How is a due date worked out after IVF?
From the embryo age and transfer date, and that ART-derived date should be used to set the due date.1 Your fertility clinic can confirm your dates.
I have a due date from my scan. How do I use it in the calculators?
Enter it directly: the weeks calculator accepts a due date and shows how far along you are, and the conception calculator accepts a due date too. Using your scan due date keeps the results consistent with your scan dating.
Why might two calculators give different due dates?
Tools can apply different rules — some adjust for cycle length, for example. This site uses the standard 280-day convention1 and doesn’t adjust for cycle length, because no adjustment formula has been verified against the sources it uses. If your healthcare team has given you a due date, use theirs.
What if I don’t know the date of my last period?
Speak to a midwife or GP.2 An early ultrasound is the most accurate way to establish how far along you are.1
Accuracy and changes
How accurate is an estimated due date?
It’s an estimate, not a prediction of the day. In one study of 18,708 spontaneous-labour births, only 5% happened on the estimated date,3 and approximately 66% happened within 7 days of it.3 Read more: How accurate a due date is.
Is my due date the exact day my baby will arrive?
Not necessarily. The due date marks 40 weeks 0 days, while pregnancy normally lasts from 37 to 42 weeks.2
Can an ultrasound change the due date?
It can. Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.1 Healthcare professionals compare it with the LMP date and change the due date only when the difference is larger than a threshold that depends on how far along the pregnancy is.1 Read more: Ultrasound vs LMP dating.
Can my due date keep changing?
It shouldn’t. Once set from the LMP and/or the first accurate ultrasound, later changes should be reserved for rare circumstances.1
Pregnancy weeks and term
How many weeks is a typical pregnancy?
A singleton pregnancy lasts an average of 40 weeks (280 days) from the first day of the last period.4 The NHS says pregnancy normally lasts from 37 to 42 weeks.2
How many weeks pregnant am I?
Count the days since the first day of your last period and divide by 7: completed weeks, then the remaining days — for example, 12 weeks 3 days.5 You can find out how many weeks pregnant you are today. Read more: How pregnancy weeks are counted.
Why am I counted as pregnant before conception?
Because pregnancy is dated from the last period and the convention assumes ovulation on day 14,1 the first two weeks counted come before conception.
What does “40 weeks 0 days” mean?
It’s the estimated due date: 280 days after the first day of the last period, counted in completed weeks and days.15
Is 37 weeks full term?
37 weeks 0 days to 38 weeks 6 days is ACOG’s early-term category; full term is 39 weeks 0 days to 40 weeks 6 days.4
What if my due date passes?
Passing the due date on its own is within the normal range, which runs to 42 weeks.2 ACOG calls 41 weeks 0 days to 41 weeks 6 days late term and 42 weeks 0 days onwards postterm.4 Your midwife or doctor can talk you through what happens next for you.
Is a due date 39 or 40 weeks?
The due date is 40 weeks 0 days. ACOG’s full-term category runs from 39 weeks 0 days to 40 weeks 6 days.4
How many days is a term pregnancy?
The WHO defines term as 37 to less than 42 completed weeks: 259 to 293 days from the first day of the last period.5
What does preterm mean?
Born before 37 weeks of pregnancy are completed.6
Conception
Can I estimate conception from a due date?
You can get an estimate by convention. The convention assumes ovulation on day 14 of a 28-day cycle,1 which places an estimated conception date 266 days before the due date. You can estimate a conception date from your due date. Read more: How to estimate a conception date.
Can I know my exact conception date?
Not from a calculation. The convention doesn’t account for irregular cycles or variable ovulation timing,1 so the result isn’t the exact date conception happened.
Can a conception date establish paternity?
No. An estimated conception date can’t establish paternity and shouldn’t be used to determine it.
Irregular periods
What if my periods are irregular?
The standard calculation assumes a regular 28-day cycle, so it doesn’t account for irregular cycle length or variable ovulation timing.1 An early scan can establish or confirm how far along you are.1 Read more: Due dates with irregular periods.
Does this site adjust for cycle length?
No. No cycle-length adjustment formula has been verified against the sources this site uses, so the calculators use the standard convention and say what it assumes.
Terms
What does EDD mean?
EDD stands for estimated due date, the term the NHS uses for the result of a due date calculation.2 Read more: What EDD means.
What does LMP mean?
The first day of your last menstrual period — the starting point for pregnancy dating.5
What is gestational age?
How far along a pregnancy is, measured from the first day of the last normal menstrual period and expressed in completed weeks and days.5
About the calculators on this site
Are the calculators private?
Yes. The calculators run in your browser, and the dates you enter aren’t sent to this website or stored.
Is this medical advice?
No. This site gives general information and estimates. Your midwife or doctor can confirm your dates.
Medical statements in this article are linked to the numbered sources listed below: published clinical guidance and peer-reviewed research. It has not been reviewed by a named clinician. Read how content is written in our editorial policy, and see our medical disclaimer.
General information, not medical adviceThis article explains how pregnancy dating works in general. It can’t take your own history into account. Your midwife or doctor can confirm your dates.
Work out your dates
Free, private calculators that run in your browser.
Calculate your estimated due date