How accurate is a due date from my last period?

It is a reasonable estimate and a poor promise. Roughly 4 in 100 babies arrive on the exact date, while the large majority arrive somewhere between 37 and 41 weeks. The rule assumes textbook ovulation on day 14, so anything that moves ovulation moves the real date without moving the calculated one. An early ultrasound is considerably more accurate.

Why does pregnancy start two weeks before conception?

Gestational age is counted from the first day of the last menstrual period because that is the last event with a date most people can name. Conception normally happens about two weeks later, so on the day you conceive you are already described as two weeks pregnant. Every clinical milestone, scan window and screening cut-off uses this convention.

What if I do not know the date of my last period?

Ask for a dating scan. A crown rump length measurement taken between roughly 8 and 13 weeks dates a pregnancy to within a few days without needing any dates from you. This is also the right route if your cycles are irregular, if you conceived soon after stopping hormonal contraception, or if you were breastfeeding.

Does a longer cycle really change the due date?

Yes. A longer cycle usually means ovulation later in the cycle, so conception and birth both shift later by about the same number of days. A 35 day cycle pushes the estimate roughly a week past the plain Naegele result. Short cycles work the same way in reverse and bring the date forward.

Should I use the calculator date or the scan date?

Use the scan date. Clinicians generally replace an LMP based date when a first trimester scan disagrees by more than about five to seven days, because early fetal growth is very consistent between pregnancies. Once a date is set from an early scan it is normally kept for the rest of the pregnancy, even if later scans suggest otherwise.

What counts as full term and post term?

Early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks, and post term begins at 42 weeks 0 days. Monitoring usually increases past 41 weeks and induction is commonly discussed before 42 weeks. Your maternity team sets the local policy.

Pregnancy Due Date Calculator

Why use this calculator

The due date is the hook every other pregnancy decision hangs off: when scans are booked, when screening tests are valid, when parental leave starts and when a birth counts as early, term or post term. Working it out by hand is fiddly because pregnancy is dated from the first day of your last period rather than from conception, which makes you two weeks pregnant on the day you conceive.

This calculator does that arithmetic in one step, adjusts it for a cycle that is not the textbook 28 days, and shows the trimester boundaries and the full term window alongside the single date. It is an estimate rather than an appointment. Only about 4 in 100 babies arrive on the estimated day, and a dating ultrasound in the first trimester is more accurate than any calculation from dates you remember.

How to use it

Enter the day, month and year of the first day of your last menstrual period, which means the first day of real bleeding rather than spotting and not the day bleeding stopped.
If you track cycles in an app, that start date is the one the app marks as day one.
Then enter your average cycle length, counted from the first day of one period to the first day of the next, which is commonly 24 to 35 days rather than exactly 28.
If your cycles vary a lot, enter the length you see most often and treat the result as a wider window.
Read the due date together with the full term window underneath it, because the window is the honest answer and the single date is the shorthand.

What each result means

Estimated Due Date: The 40 week mark counted from your last period. Treat it as the centre of a window rather than a booking.
Estimated Conception Date: Roughly when fertilisation happened, assuming ovulation two weeks before your next period was due. Sperm survival widens this by several days.
Current Gestational Age: How many weeks and days pregnant you are today, counted from the last period the way a midwife or obstetrician counts it.
Current Trimester: Which third of the pregnancy today falls in, using the obstetric boundaries of 13 weeks 6 days and 27 weeks 6 days.
Time Left Until the Due Date: Weeks and days remaining to the 40 week mark, or how far past it you are if the date has already passed.
End of First Trimester: The last day of the first trimester, 13 weeks 6 days. Miscarriage risk drops sharply once this point is reached.
End of Second Trimester: The last day of the second trimester, 27 weeks 6 days, after which the third trimester and more frequent appointments begin.
Full Term Window: The 37 to 41 week span most births fall inside. Arriving anywhere in this window is normal, not early or late.
Cycle Length Adjustment: How many days your cycle length moved the due date compared with the textbook 28 day assumption behind Naegele's rule.

The formula

Naegele's rule: estimated due date = first day of the last menstrual period plus 280 days, which is 40 weeks

This calculator adds a cycle adjustment, so due date = LMP + 280 + (cycle length minus 28) days

Because a longer cycle means later ovulation and a later birth.

Gestational age in days = today minus LMP minus the same cycle adjustment

Trimester boundaries follow obstetric convention: the first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. Full term is 37 weeks 0 days to 40 weeks 6 days, and post term begins at 42 weeks 0 days.

How this number is used in practice

In practice the date you are given at your first appointment usually comes from an ultrasound, not from a calculation. A crown rump length measurement between about 8 and 13 weeks dates a pregnancy to within a few days, because early fetal growth is remarkably consistent between pregnancies.

Clinicians generally change a date based on the last period if the scan disagrees by more than about five to seven days in the first trimester. Later scans are much weaker for dating, since by the third trimester babies vary in size for reasons that have nothing to do with how old they are, so a date set early is normally kept for the rest of the pregnancy.

The single date matters most for the decisions that sit at the edges. Induction of labour, monitoring for a post term pregnancy, and the threshold at which a birth is treated as preterm all key off the dated gestational age rather than off a guess.

It also drives the paperwork: parental leave start dates, employer notification deadlines and insurance waiting periods are all written against an expected date of confinement. If the date moves after a scan, tell your employer and your fund, because the earlier figure may already be sitting in their system.

None of this is medical advice, and any bleeding, severe pain, reduced fetal movement or concern about dates belongs with your midwife, GP or maternity unit rather than with a calculator.

Worked examples

Last period started 1 January 2026, 28 day cycle
Due 8 October 2026, with the first trimester ending 8 April 2026 and full term from 17 September 2026.
Last period started 15 March 2026, longer 32 day cycle
Due 24 December 2026, 4 days later than the unadjusted 20 December 2026 that Naegele's rule alone would give.
Last period started 10 June 2026, shorter 24 day cycle
Due 13 March 2027, 4 days earlier than the unadjusted date because ovulation happens earlier on a short cycle.

When people use it

Booking dating, screening and anatomy scans in the right weeks
Working out how many weeks pregnant you are for a form or an appointment
Planning parental leave start dates and employer notification deadlines
Checking whether a birth would count as preterm, term or post term
Sanity checking a date given to you against the dates you remember

Tips

Use the first day of real bleeding, not spotting, and not the day the period ended.
A dating scan in the first trimester beats any calculation from remembered dates, so let the scan win.
Only about 4 in 100 babies arrive on the estimated date, so plan around the 37 to 41 week window instead.
If your cycles swing by more than a week, treat the due date as a range rather than a day.
Write the date down with the source next to it, since a scan date and an LMP date often differ by several days.
Tell your employer and your health fund if the date changes after a scan, because the old date is already in their systems.
Irregular cycles, recent hormonal contraception or breastfeeding all make an LMP based date much less reliable.

What it assumes, and where it stops being accurate

Naegele's rule assumes a regular cycle with ovulation around day 14, which many people do not have.
The cycle adjustment moves the due date day for day, which approximates but does not measure actual ovulation timing.
It is not accurate after recent hormonal contraception, during breastfeeding, or with irregular or very long cycles.
It does not apply to IVF pregnancies, where the transfer date and embryo age give a far more precise due date.
Twins and higher order multiples are typically delivered earlier than the single baby full term window shown here.
All dates are calculated in UTC and rounded to whole days, so a result can sit a day either side of a local calendar date.

Terms used on this page

Gestational age: How far along a pregnancy is, counted in weeks and days from the first day of the last menstrual period.
Naegele's rule: The 280 day rule for estimating a due date from the first day of the last menstrual period.
Estimated due date: The 40 week mark, also written EDD or estimated date of confinement, used as the reference for every other milestone.
Crown rump length: The head to bottom measurement taken at an early ultrasound, the most accurate way to date a pregnancy.
Full term: A birth between 37 weeks 0 days and 40 weeks 6 days of gestation, the window most births fall in.

Questions people ask

How accurate is a due date from my last period?: It is a reasonable estimate and a poor promise. Roughly 4 in 100 babies arrive on the exact date, while the large majority arrive somewhere between 37 and 41 weeks. The rule assumes textbook ovulation on day 14, so anything that moves ovulation moves the real date without moving the calculated one. An early ultrasound is considerably more accurate.
Why does pregnancy start two weeks before conception?: Gestational age is counted from the first day of the last menstrual period because that is the last event with a date most people can name. Conception normally happens about two weeks later, so on the day you conceive you are already described as two weeks pregnant. Every clinical milestone, scan window and screening cut-off uses this convention.
What if I do not know the date of my last period?: Ask for a dating scan. A crown rump length measurement taken between roughly 8 and 13 weeks dates a pregnancy to within a few days without needing any dates from you. This is also the right route if your cycles are irregular, if you conceived soon after stopping hormonal contraception, or if you were breastfeeding.
Does a longer cycle really change the due date?: Yes. A longer cycle usually means ovulation later in the cycle, so conception and birth both shift later by about the same number of days. A 35 day cycle pushes the estimate roughly a week past the plain Naegele result. Short cycles work the same way in reverse and bring the date forward.
Should I use the calculator date or the scan date?: Use the scan date. Clinicians generally replace an LMP based date when a first trimester scan disagrees by more than about five to seven days, because early fetal growth is very consistent between pregnancies. Once a date is set from an early scan it is normally kept for the rest of the pregnancy, even if later scans suggest otherwise.
What counts as full term and post term?: Early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks, and post term begins at 42 weeks 0 days. Monitoring usually increases past 41 weeks and induction is commonly discussed before 42 weeks. Your maternity team sets the local policy.

Use this with

Pregnancy Due Date Calculator answers one part of the question. These pick up where it stops: Ovulation Calculator, Age Calculator, Date Calculator and BMI Calculator.

Where these figures come from

Works anywhereLast reviewed 2026-09·How we build and check calculators

This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.

NHS, Pregnancy. Due date, antenatal appointment and scan timing
American College of Obstetricians and Gynecologists, Pregnancy. Trimester boundaries and term definitions
World Health Organization, Maternal health. Antenatal care and gestational age standards

Pregnancy Due Date Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.