Pregnancy

Your Due Date: How It's Set, How Sure It Is

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Your due date starts from the first day of your last period plus 280 days, roughly 40 weeks, using Naegele's rule. An early ultrasound refines it by measuring the baby. It is an estimate, not a deadline, and only about 1 in 20 babies arrive on the exact date.

Last updated: July 2026

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How is a due date first calculated?

The starting point is the first day of your last menstrual period, not the day you conceived. A shortcut called Naegele's rule adds 280 days, about 40 weeks, to that first day to set an estimated due date 2. The method assumes a regular 28-day cycle with ovulation around day 14, which is why it is only a starting estimate. Because it counts from the period rather than conception, the first two weeks of those 40 are technically before pregnancy begins. According to prenatal care overviews, this last-period calculation is how most due dates are first assigned before any scan 2. It gives a workable anchor that later measurements can adjust.

Why does an early ultrasound change the date?

An early ultrasound measures the baby directly, which often sharpens a date built on period math. In the first trimester, the length of the baby from crown to rump predicts gestational age within a few days, so a dating scan between 11 and 14 weeks is more reliable than last-period dates for many people 1. When the scan and the period estimate disagree by more than a set number of days, clinicians usually adopt the scan date. This matters most when cycles are irregular, when a period was misremembered, or shortly after stopping birth control, when ovulation timing can be uncertain. According to antenatal guidance, early scan dating is the preferred way to set the due date 1.

How accurate is a due date?

A due date marks the middle of a normal range, not a guaranteed delivery day. Full-term birth spans roughly 37 to 42 weeks, so babies routinely arrive in the weeks on either side of the date on the calendar. Only a small share of babies, often cited as around 1 in 20, are born on the exact predicted day. Cycle length adds to the spread: someone with longer or irregular cycles may ovulate later than day 14, nudging a true date past the estimate, which is common for teenagers with newer cycles or anyone with irregular periods. Knowing this can ease the pressure many people feel as the marked date passes without contractions.

What if I go past my due date?

Passing your due date is common and, on its own, not a problem. Many pregnancies continue a week or more beyond 40 weeks, and clinicians add monitoring as the days accumulate. Going to or beyond 41 weeks is when induction is usually discussed, because inducing at that point lowers the small risk of stillbirth compared with waiting 3. Extra checks such as a non-stress test or a scan of the fluid around the baby help confirm all is well in the meantime. Reviewing the plan at prenatal visits keeps decisions grounded in your specific dating rather than the calendar alone. An accurate early date is what makes these later choices sound.

When your due date needs a clinician's judgment

A due date set on shaky information is where a clinician's judgment matters most. If your cycles were irregular, your last period is uncertain, or you tested positive later than expected, an early scan is the way to firm up the timeline 1. Learning how soon a pregnancy test turns positive can help you understand your own dates, and early spotting is a reason to have dating confirmed. Bleeding, a severe headache, or reduced movement near term are reasons to be seen the same day regardless of the date. Gale can help you organize the details your clinician needs to set the most accurate estimate.

Common questions

Usually not directly. The standard calculation counts 280 days from the first day of your last period, which is about two weeks before conception. An early ultrasound can refine this by measuring the baby, but the period date is the common starting point.

For most people, an early first-trimester ultrasound is more accurate, because it measures the baby directly. When the scan and period dates disagree by more than a set margin, clinicians generally use the scan date, especially with irregular cycles.

The due date is the midpoint of a normal range, not a fixed day. Full-term birth spans about 37 to 42 weeks, and factors like cycle length shift timing, so only around 1 in 20 babies are born on the exact predicted date.

Passing your due date is common and not a problem on its own. Clinicians add monitoring as the days pass and usually discuss induction at or beyond 41 weeks. Reduced movement or bleeding, though, is a reason to be seen promptly.

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When late-pregnancy signs need prompt review

  • —A drop in your baby's usual movements near or past your due date is a reason to contact your obstetric clinician the same day.
  • —Vaginal bleeding or a gush or trickle of fluid is a reason to seek clinician review promptly.
  • —A severe headache, vision changes, or sudden swelling of the hands and face is a reason to be evaluated urgently.
  • —Reaching or passing 41 weeks is a reason to discuss monitoring and induction options with your care team.

This article is general health education, not medical advice. How your due date is set and when to act on it is decided by your obstetric clinician or midwife using your dating scan and history.

References

  1. 1.National Institute for Health and Care Excellence (2024). Antenatal care (NG201). National Institute for Health and Care Excellence (NICE). link ✓Antenatal care guidance recommending an early ultrasound (dating scan) around 11 to 14 weeks to estimate gestational age, which is more accurate than last-menstrual-period dating for many pregnancies.
  2. 2.Office on Women's Health (U.S. HHS) (2025). Prenatal care. Office on Women's Health (womenshealth.gov), U.S. HHS. link ✓Patient-facing overview explaining that a due date is estimated from the last menstrual period and counted across about 40 weeks of pregnancy.
  3. 3.Middleton P, Shepherd E, Morris J, Crowther CA, Gomersall JC (2020). Induction of labour at or beyond 37 weeks' gestation. Cochrane Database of Systematic Reviews. doi:10.1002/14651858.CD004945.pub5 ✓Review showing that inducing labour at or beyond 41 weeks is associated with fewer perinatal deaths than expectant management, informing decisions once a pregnancy passes its due date.

3 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — every citation independently verified. Editorial policy

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