Pregnancy Due Date Calculator

Understanding the Pregnancy Due Date Calculator

A due date calculator adds 280 days to the first day of your last menstrual period. It is the standard method and it rests on an assumption worth knowing: that you ovulated on day 14 of a 28-day cycle, which is true for a minority of pregnancies.

How it actually works

Enter the year, month, and day your last menstrual period began. The calculator adds 280 days, which is 40 weeks, and reports the estimated due date alongside how many weeks pregnant you currently are. A last period beginning 15 June 2026 gives a due date of 22 March 2027.

The deeper context most people miss

Dating from the last period means you are counted as two weeks pregnant at conception, which confuses almost everyone initially. The convention exists because the last period is a date most people can identify while ovulation usually is not, and every gestational milestone is expressed on that basis rather than from conception.

Naegele's rule, its assumptions, and why ultrasound supersedes it

The 280-day calculation is known as Naegele's rule, attributed to a nineteenth-century German obstetrician, and it assumes a regular 28-day cycle with ovulation on day 14. Both assumptions fail frequently. Cycle length varies substantially between women and between cycles in the same woman, and studies tracking ovulation directly have found that only a minority of women ovulate exactly on day 14, with the timing of ovulation being the main source of variability while the luteal phase after ovulation stays relatively consistent. A woman with a 35-day cycle ovulates around day 21, so her true gestational age is about a week behind what Naegele's rule suggests, and dating her from her last period would put her a week ahead of where she actually is. This matters clinically because gestational age drives decisions about screening timing, growth assessment, induction, and management of preterm or post-term pregnancy. For this reason, current obstetric guidance is that ultrasound dating takes precedence over menstrual dating in most circumstances. A first-trimester ultrasound measuring crown-rump length dates a pregnancy to within about five to seven days, which is considerably more accurate than menstrual dating, and if the ultrasound date differs from the menstrual date by more than about a week in the first trimester, the ultrasound date is generally adopted. Accuracy of ultrasound dating declines as pregnancy progresses, since growth becomes more variable, which is why an early scan is more useful for dating than a later one.

A worked example: how much the due date actually means

A due date of 22 March 2027 sounds like an appointment, and it is better understood as the centre of a distribution. Only around 4% of babies are born on their due date. Roughly half of first-time mothers deliver after it, and the distribution is wide: term is now defined across a range, with early term at 37 to 38 weeks 6 days, full term at 39 to 40 weeks 6 days, late term at 41 weeks, and post-term beyond 42. That range exists because outcomes differ meaningfully across it, which is why the terminology was revised to discourage treating anything from 37 weeks as equivalent. Practically, planning around a due date as though it were fixed causes avoidable stress, and it is more useful to think of a two-week window either side. Several factors shift the distribution: first pregnancies tend to go slightly longer than subsequent ones, and there is some evidence of variation with ethnicity and maternal age. What the due date genuinely determines is the timing of care rather than the day of birth: screening tests have windows tied to gestational age, growth is assessed against it, and decisions about induction for post-term pregnancy are made relative to it, with most guidelines recommending offering induction somewhere between 41 and 42 weeks given rising stillbirth risk beyond that point.

Deciding what to do when dates are uncertain

Several situations make menstrual dating unreliable and warrant early ultrasound. Irregular cycles are the most common: if cycle length varies by more than a few days, the day-14 ovulation assumption fails and menstrual dating can be off by a week or more. Conception shortly after stopping hormonal contraception is another, since cycles frequently take months to regularise and the first ovulation may occur without a preceding period. Breastfeeding suppresses ovulation variably and fertility can return before periods do, meaning conception can occur with no last period to date from. Bleeding in early pregnancy is sometimes mistaken for a period, producing a date several weeks too late. Conception during a cycle where a period was missed or unusually light similarly misleads. Assisted conception is the opposite case: with IVF, the date of embryo transfer and the embryo's age at transfer are known precisely, so dating is exact and neither menstrual nor ultrasound dating is needed. In any situation where the date is uncertain, an early scan resolves it, and the earlier it is done the more accurate it is. It is worth raising uncertainty explicitly with a midwife or doctor rather than reporting an approximate last period date as though it were reliable, since the dating decision affects care throughout.

Why gestational age drives so much of maternity care

Almost every element of antenatal care is scheduled against gestational age, which is why getting it right matters more than the due date itself. Screening has narrow windows: combined first-trimester screening for chromosomal conditions is performed around 11 to 14 weeks and is invalid outside that range, while the anomaly scan sits around 18 to 21 weeks. Non-invasive prenatal testing requires sufficient fetal fraction, generally from around 10 weeks. Glucose tolerance testing for gestational diabetes is typically offered around 24 to 28 weeks. Anti-D prophylaxis for rhesus-negative women follows a schedule. Growth assessment compares measurements against gestational age norms, so incorrect dating can produce apparent growth restriction or macrosomia where none exists, potentially triggering unnecessary intervention or missing genuine problems. Decisions at the edges of viability, around 22 to 24 weeks, depend critically on accurate dating. Steroid administration for fetal lung maturation in threatened preterm birth is timed against gestation. Post-term management, including surveillance and induction, is triggered by gestational age. Given this, the single most useful thing about a due date is not the day itself but the gestational age framework it establishes, and this is why obstetric practice is firm about establishing dates early and not revising them later without good reason, since changing dates mid-pregnancy disrupts the entire schedule of care.

Variations: cycle length adjustment, ultrasound, and IVF dating

Naegele's rule can be adjusted for cycle length by adding the difference between your cycle and 28 days, so a woman with a 35-day cycle would add seven days to the calculated date. This is a reasonable correction for someone with regular but non-standard cycles, though it still assumes a consistent luteal phase. Ultrasound dating using crown-rump length in the first trimester is accurate to roughly five to seven days and is the current standard where available. Second-trimester dating using multiple biometric measurements is less accurate, around seven to ten days, and third-trimester dating is poor enough that it is generally not used to revise established dates. IVF pregnancies are dated exactly from transfer date and embryo age, since both are known. Some models attempt to account for individual variation in cycle length, ethnicity, parity, and maternal characteristics to produce a personalised estimate, with research suggesting mean gestation length varies modestly between groups. For twins and higher-order multiples, gestation is typically shorter and delivery is planned rather than awaited, so the due date functions differently.

Understanding your due date

Treat it as the centre of a range rather than a fixed date, since only around 4% of babies arrive on it and roughly half of first-time mothers deliver after it. Have an early ultrasound where possible, since first-trimester dating by crown-rump length is accurate to within about five to seven days and takes precedence over menstrual dating in current guidance. Mention explicitly if your cycles are irregular, if you conceived shortly after stopping contraception, if you were breastfeeding, or if you had bleeding that might not have been a period, since all of these make menstrual dating unreliable. Understand that dating from the last period means you are counted as two weeks pregnant at conception, which is a convention rather than an error. And recognise that gestational age drives the timing of screening, growth assessment, and decisions about induction, which is why establishing it accurately early matters more than the due date itself.

What people get wrong

• Treating the due date as a fixed appointment, when only around 4% of babies are born on it and roughly half of first pregnancies go past it.
• Relying on menstrual dating with irregular cycles, when the day-14 ovulation assumption fails and dating can be off by a week or more.
• Assuming conception occurred on the due date calculation's start point, when dating from the last period means you are counted as two weeks pregnant at conception.
• Expecting late-pregnancy ultrasound to revise dates reliably, when accuracy declines as growth becomes more variable and established early dates are generally not changed.

Where the math comes from

Due Date = First day of last menstrual period + 280 days, which is Naegele's rule, assuming a 28-day cycle with ovulation on day 14. Current gestational age is calculated as the weeks elapsed since the last menstrual period. Ultrasound dating by crown-rump length in the first trimester is accurate to roughly five to seven days and takes precedence over menstrual dating in current obstetric guidance.

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