How accurate is a due date?

About 4% of babies are born on the estimated date. Roughly half arrive within a week of it and about 90% within two weeks. A first-trimester ultrasound dates a pregnancy to within about five days, which is the most accurate method available; an LMP calculation with a regular cycle is close behind, and less reliable with irregular cycles.

Why is pregnancy dated from my last period rather than conception?

Because the date of a period is something you can know and the date of conception usually is not. The convention adds about two weeks at the front, which is why you are counted as four weeks pregnant when a test first turns positive despite conception being roughly two weeks earlier. Actual gestation is about 38 weeks, not 40.

What if my cycle is not 28 days?

Then the standard calculation is wrong by the difference, and you should enter your actual cycle length. Ovulation occurs roughly 14 days before your next period, so on a 35-day cycle it happens about a week later than the textbook assumption — and your due date is about a week later too. This is a leading reason early ultrasounds reassign due dates.

Can the due date change?

Yes, and it commonly does after an early scan. ACOG advises redating when the ultrasound differs from the LMP estimate by more than about five to seven days in the first trimester. Once a due date has been set from an early scan it is not normally changed again, because later scans measure growth rather than age.

What happens if I go past my due date?

Nothing immediately — it is normal, and about 60% of first-time mothers are still pregnant on the day. Monitoring usually increases after 41 weeks, and induction is commonly offered somewhere between 41 and 42 weeks, since risks rise beyond that point. The specifics vary by country, by provider and by your circumstances, and are a conversation with your care team rather than a calculation.

When does each trimester start and end?

Using ACOG boundaries: the first trimester runs from 0w0d through 13w6d, the second from 14w0d through 27w6d, and the third from 28w0d until birth. Some sources draw the lines a week differently, which is why you will occasionally see conflicting answers for the same date.

Pregnancy Due Date Calculator

Estimate your baby's due date based on your last menstrual period. See your current week, trimester milestones, and conception date.

First Day of Last Menstrual Period (LMP)

Average Cycle Length (days)

28 is the textbook assumption. A longer cycle means later ovulation and a later due date, day for day.

This tool is for informational purposes only and does not constitute medical advice. Consult a healthcare professional for advice specific to your situation.

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What is Pregnancy Due Date?

A due date is a single day standing in for a range. It is calculated by adding 280 days — forty weeks — to the first day of your last menstrual period, a method known as Naegele's rule after the German obstetrician who formalised it in 1812.

The oddity built into that method is worth understanding before anything else: pregnancy is dated from the last period, which is roughly two weeks before conception. So at the moment of conception you are already counted as two weeks pregnant, and the forty weeks includes a fortnight in which you were not pregnant at all. Actual gestation is about 38 weeks.

The reason for this convention is practical. The date of a period is something you can know; the date of conception generally is not. Dating from an observable event makes the estimate reproducible even though it is anatomically odd.

This is an estimate, not an appointment. Only about 4% of babies arrive on their due date, and a normal birth falls anywhere in a five-week window.

The formula — how to calculate Pregnancy Due Date

Due date = LMP + 280 days + (cycle length − 28) Gestational age = days since LMP, expressed as weeks + days Estimated conception = LMP + (cycle length − 14) Trimesters (ACOG): First 0w0d – 13w6d Second 14w0d – 27w6d Third 28w0d onward

LMP: = the first day of your last menstrual period, not the last day
Cycle length: = your usual interval between periods; 28 days is the textbook assumption, not a rule
280 days: = forty weeks — the convention, which corresponds to about 38 weeks of actual gestation

The cycle adjustment matters more than most calculators admit. Ovulation happens roughly 14 days before the next period, so on a 35-day cycle it occurs around day 21 rather than day 14 — and the due date moves a week later.

Step-by-step example

01Last period began 1 March 2026, with a typical 28-day cycle.
02Due date: 1 March + 280 days = 6 December 2026.
03Estimated conception: 1 March + 14 days = 15 March.
04First trimester ends 6 June (13w6d); second ends 12 September (27w6d).
05Now change one fact: the same LMP but a 35-day cycle. Ovulation falls around day 21 rather than day 14, so the due date moves seven days later, to 13 December 2026.
06That week is not trivial. It affects when you are offered screening tests, when you would be considered post-term, and when induction might be discussed. It is the most common reason an early ultrasound reassigns a due date.

Why the ultrasound date usually wins

If an early scan gives a different due date from your LMP calculation, the scan is generally the one used — and that is the right call.

In the first trimester, embryos grow at a highly consistent rate, so measuring crown-rump length dates a pregnancy accurately to within about five days. The LMP method depends on remembering a date correctly and on assumptions about your cycle that may not hold.

ACOG guidance is that a first-trimester ultrasound should redate the pregnancy when it differs from the LMP estimate by more than about five to seven days, depending on how early the scan was.

Accuracy falls away as pregnancy progresses. By the third trimester, a scan can be two to three weeks out on dating, because babies of the same gestational age differ increasingly in size. This is why an early scan is used for dating and later scans for growth — they answer different questions.

If your periods are irregular, or you conceived while on hormonal contraception or shortly after stopping it, the LMP figure is a rough starting point at best and the scan should be treated as definitive.

IVF dating is exact

After IVF, the date of transfer and the age of the embryo are both known precisely, so the due date is not an estimate at all. For a day-5 blastocyst transfer, the due date is 261 days from transfer. This is the one situation where the number is not approximate — and it is why an IVF due date should not be recalculated from an LMP.

What "term" actually means

The single due date obscures a more useful set of thresholds. In 2013 ACOG replaced the loose category of "term" with four defined bands, precisely because babies born at 37 weeks and at 40 weeks have measurably different outcomes and lumping them together hid that.

Gestational age categories

Category	Weeks	Notes
Preterm	Before 37w0d	Increased risk; extremely preterm is before 28 weeks
Early term	37w0d – 38w6d	Slightly higher rate of respiratory and feeding difficulty
Full term	39w0d – 40w6d	The lowest-risk window for most pregnancies
Late term	41w0d – 41w6d	Monitoring usually increases
Post term	42w0d and beyond	Rising risk; induction is generally offered before this

A due date is the start of week 40. Passing it is normal — about 60% of first-time mothers are still pregnant on their due date.

How wide the real distribution is

The estimate is a central point in a distribution, and knowing the shape of it takes some of the anxiety out of the last few weeks.

Roughly 4% of babies are born on their due date. About half arrive within a week either side, and around 90% within two weeks. Spontaneous labour clusters slightly after the due date rather than before it, which is why so many first pregnancies feel overdue.

First pregnancies run longer on average — commonly cited as about five days beyond subsequent ones. Longer previous pregnancies predict longer ones again, which suggests a genuine individual tendency rather than pure chance.

A frequently cited study of naturally conceived pregnancies found the length of gestation varied by 37 days across women even after excluding preterm births. Against that spread, a due date accurate to within a few days is doing well.

The practical implication is about planning rather than medicine: arranging leave, childcare or travel around a single date is optimistic. Planning around a two-week window is realistic.

What the weeks are counting

Gestational age —: measured from the LMP — the number quoted in every appointment and on every scan. "12 weeks pregnant" means 12 weeks of gestational age.
Fetal age —: measured from conception, and about two weeks less. Sometimes used in embryology texts, which is a common source of confusion when reading about development.
Weeks and days —: clinicians write 24w3d, meaning 24 completed weeks and 3 days. It is not 24.3 weeks, and reading it that way misses a day or two.
Viability —: the point at which survival outside the womb becomes possible, generally considered around 24 weeks and improving with neonatal care. It is a probability that rises steeply week by week, not a threshold.
Estimated date of delivery (EDD) —: the clinical term for the due date. "Estimated" is doing real work in that phrase.

Common mistakes to avoid

Using the last day of your period rather than the first day. It shifts the estimate by the length of your period.
Assuming a 28-day cycle when yours is consistently longer or shorter.
Treating an LMP calculation as more authoritative than an early ultrasound. It is not.
Reading 24w3d as 24.3 weeks.
Recalculating an IVF due date from an LMP, which discards the one precisely known date.
Planning leave or travel around the exact due date rather than a window.
Assuming a due date passed means something is wrong. It is normal, and most first babies arrive after it.

Frequently asked questions

Sources & references

Written and fact-checked by the CalcProLabs Editorial Team against ACOG trimester definitions and gestational age conventions. Read our calculation methodology and editorial policy.

Last updated August 7, 2026