Pregnancy Guide · Medically Reviewed

The Complete Due Date Guide

Everything pregnancy dating actually means: how your due date is calculated, how accurate it really is, what changes it, and what happens if your baby arrives before or after it.

Medically reviewed Updated June 2026 Based on ACOG & NHS guidance 11-minute read

Quick answer

Your due date is an estimate, not a deadline — calculated as 280 days from the first day of your last period, or from a clinical conception or ultrasound date. Only about 1 in 20 babies are born on that exact day; most arrive within the five-week “term” window between 37 and 42 weeks. Use the calculator to get your number, then come back here for the full picture of what it means.

Calculate your due date

What a due date actually is

Your estimated due date (EDD) — sometimes written as EDC, “estimated date of confinement” — is the calendar date on which a pregnancy reaches 40 completed weeks, counted from the first day of your last menstrual period (LMP). It is not a forecast of your baby’s actual birthday. It is a clinical reference point: a single day in the middle of a normal five-week range that your prenatal team uses to schedule tests, track growth, and decide when a pregnancy has gone on long enough to warrant a closer look.

The convention itself is a quirk of timing. Pregnancy is dated from the LMP rather than from conception because most people can identify the first day of their last period with reasonable confidence, while the exact day of conception — which happens roughly two weeks later, around ovulation — is usually unknown. That’s also why you’re considered “4 weeks pregnant” at the point your period would have been due, even though fertilization happened only about two weeks earlier.

Due date ≠ deadline

It marks the midpoint of a normal range, not a finish line. A pregnancy that ends a week or two on either side of this date is still considered completely typical.

Gestational age ≠ fetal age

“Weeks pregnant” (gestational age) is counted from the LMP. Your baby’s actual developmental age is about two weeks younger than that number.

How your due date is calculated

Clinicians use one of four methods to set a due date, depending on what information is available. Each one adds a fixed number of days to a known reference point, derived from the average 280-day (40-week) length of human pregnancy.

MethodFormulaTypical accuracyBest used when
LMP (Naegele’s rule) First day of LMP + 280 days ± 5–7 days Periods are regular and the LMP date is known
First-trimester ultrasound Crown-rump length, measured 8–13 weeks ± 3–5 days LMP is unknown, cycles are irregular, or it differs from LMP dating by >5–7 days
Known conception date Conception date + 266 days ± 1–3 days Ovulation was tracked precisely (monitor, BBT, LH strips)
IVF embryo transfer Day-5 transfer + 261 days, or Day-3 transfer + 263 days ± 1 day Always, for IVF pregnancies — the embryo’s exact age is known

The American College of Obstetricians and Gynecologists (ACOG) treats a first-trimester ultrasound as the gold standard whenever it’s available, because it measures the embryo directly rather than relying on assumptions about cycle length and ovulation timing. Once a due date is established this way — usually at the first prenatal visit — it’s generally fixed for the rest of the pregnancy and isn’t recalculated from later scans.

Get your exact date

Run any of these four methods instantly with the Due Date Calculator — including cycle-length adjustment for LMP dating.

Open the calculator

How accurate is your due date, really?

Multiple population studies — including research from the National Institute of Environmental Health Sciences tracking precisely-timed conceptions — find that only around 4–5% of babies are born on their exact estimated due date. That’s not a flaw in the math; it reflects genuine, healthy biological variation in how long different pregnancies last, even when dating is precise. Most births happen within roughly two weeks of the EDD, and the large majority fall inside the 37–42 week “term” range.

Infographic

When babies are typically born

Approximate share of births by gestational-age category — population averages, not a prediction for any individual pregnancy.
Bar chart of birth timing by gestational age category Preterm, before 37 weeks, about 10 percent. Early term, 37 to 38 weeks, about 26 percent. Full term, 39 to 40 weeks, about 57 percent, the category containing the estimated due date. Late term, 41 weeks, about 5 percent. Postterm, 42 weeks and beyond, about 2 percent. Preterm Before 37 wks ~10% Early term 37–38 wks ~26% Full term 39–40 wks ~57% ↑ your due date (week 40) falls in this band Late term 41 wks ~5% Postterm 42+ wks ~2%
Figure 1. Approximate distribution of birth timing relative to gestational age. Individual figures vary by source year and population studied; treat as a general pattern, not a forecast.

Two things make a real difference to how tight that ± window is: which dating method was used and how early in pregnancy it was applied. A first-trimester ultrasound is precise because the embryo’s size is still tightly correlated with age; by the third trimester, individual growth variation makes ultrasound dating far less reliable — which is why ACOG guidance says a due date, once set early, should rarely be moved based on a later scan.

What can change your due date

A due date isn’t necessarily set in stone the day you first hear it. A few specific situations commonly cause it to be revised:

Early ultrasound discrepancy

If a first-trimester scan differs from LMP-based dating by more than about 5–7 days, ACOG guidance favors updating the due date to match the ultrasound — it’s the more reliable of the two that early on.

Irregular or unknown LMP

If you don’t track your cycle or have irregular periods, LMP dating may be unreliable from the start — a dating ultrasound between 8–13 weeks becomes the default reference.

Cycle length outside 28 days

Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Longer or shorter cycles shift the actual ovulation day, so a good calculator adjusts the LMP estimate accordingly.

It generally won’t move again later

Once a due date is confirmed by an early scan, third-trimester ultrasounds are used to monitor growth and wellbeing — not to recalculate your dates.

Worth knowing: studies comparing dating methods have found that switching a population from LMP-only dating to routine early ultrasound dating substantially reduces how often pregnancies get mislabeled as “postterm” — because the ultrasound corrects inaccurate LMP recall before it can compound over 40 weeks.

“Term,” explained on a timeline

Obstetric care no longer treats weeks 37 through 42 as one uniform “term” window. Since 2013, ACOG and the Society for Maternal-Fetal Medicine have used four more precise labels, because research showed that outcomes — especially newborn breathing problems — quietly improve the closer a baby gets to 39 weeks.

Infographic

Weeks 35–43: where your due date sits

Your due date (week 40) falls in the middle of the full-term band — the stage associated with the best average newborn outcomes.
Timeline of pregnancy term categories from week 35 to week 43 Preterm continues through 36 weeks 6 days. Early term spans 37 weeks 0 days through 38 weeks 6 days. Full term spans 39 weeks 0 days through 40 weeks 6 days, and contains the standard due date at week 40. Late term spans 41 weeks 0 days through 41 weeks 6 days. Postterm begins at 42 weeks 0 days and continues onward. Your due date Preterm < 37 wks Early term 37–38 wks Full term 39–40 wks Late term 41 wks Postterm 42+ wks 35 37 39 40 41 42 43+
Figure 2. ACOG / SMFM term-pregnancy categories (Committee Opinion No. 579), mapped against the estimated due date. Earlier delivery within the term range is sometimes medically necessary and routinely results in healthy babies — these labels describe population averages, not individual risk.
Preterm / Postterm Early term / Late term Full term (best average outcomes)

Going past your due date

Crossing the due date itself is common and, on its own, not a medical event — it’s the boundary between “full term” and “late term,” not between “fine” and “overdue.” The terms that actually matter clinically kick in at 41 and 42 weeks:

  • Late-term pregnancy (41w0d–41w6d): Still within the normal range. Many providers begin offering more frequent monitoring — non-stress tests, amniotic fluid checks, or membrane sweeps — starting around this point.
  • Postterm pregnancy (42w0d and beyond): ACOG guidance supports offering induction between 41 and 42 weeks for pregnancies without another medical indication, specifically because population data links pregnancies extending past 42 weeks to a gradual rise in complications such as placental aging and stillbirth risk. The large majority of babies born at 41–42 weeks, however, are delivered safely with no complications at all.

Two routine tools reduce how often a pregnancy is even labeled postterm in the first place: accurate early dating (an early ultrasound corrects LMP-based miscounts before they compound) and membrane sweeping, a simple in-office procedure shown in clinical trials to lower the chance of a pregnancy extending past 41 weeks.

Medical disclaimer: This guide describes general population patterns and ACOG-aligned conventions. Your own care plan — including whether and when to discuss induction — should be made with your obstetric provider based on your individual history, monitoring results, and risk factors.

If your baby arrives before the due date

A birth before 37 weeks 0 days is classified as preterm. Clinicians break this further into sub-categories, because the medical needs of a baby born at 36 weeks are very different from one born at 26 weeks:

CategoryGestational ageTypical context
Late preterm34w0d – 36w6dMost preterm births fall here; often shorter NICU stays, if any
Moderately preterm32w0d – 33w6dTypically requires NICU support for feeding and breathing
Very preterm28w0d – 31w6dSignificant NICU care; outcomes improve markedly with each added week
Extremely pretermBefore 28w0dIntensive specialist neonatal care from birth

In the United States, roughly 1 in 10 births occurs before 37 weeks. Common contributors include multiple pregnancy (twins or more), a prior preterm birth, certain uterine or cervical conditions, infections, and chronic conditions like high blood pressure or diabetes — though in many individual cases, no single clear cause is ever identified. If you’re experiencing symptoms of possible preterm labor — regular contractions, pelvic pressure, fluid leakage, or vaginal bleeding before 37 weeks — contact your provider or maternity unit right away rather than waiting to see if it passes.

Due date myths, debunked

Few topics in pregnancy attract more folklore than the due date. Here’s what the evidence actually shows.

Myth

“First babies are always late.”

Fact

First pregnancies run, on average, only a couple of days longer than later ones — a small statistical shift, not a rule. Plenty of first babies arrive before their due date.

Myth

“A full moon brings on labor.”

Fact

Large studies analyzing thousands of birth records against lunar calendars have repeatedly found no meaningful connection between moon phase and labor onset.

Myth

“Spicy food, pineapple, or long walks can induce labor near your due date.”

Fact

No food or activity has been shown in controlled research to reliably start labor. Labor begins from a hormonal cascade your body initiates on its own timeline.

Myth

“Your due date is when your baby is ‘finished’ developing.”

Fact

Key development, especially lung maturation, continues right through the full-term window — which is exactly why ACOG recommends avoiding elective delivery before 39 weeks.

Myth

“A normal pregnancy lasts exactly 280 days from conception.”

Fact

Research tracking precisely known conception dates found that healthy pregnancy length can vary by up to five weeks — even when the conception date is certain.

Myth

“A good enough ultrasound can predict your exact delivery date.”

Fact

Ultrasound estimates gestational age, not the day labor will start. No imaging test can pinpoint your baby’s actual birthday.

Using your due date to plan ahead

Even though it isn’t a deadline, your due date is still the most useful anchor point for scheduling care and getting ready. Prenatal visits typically follow this rhythm:

Gestational ageTypical visit frequency
Weeks 4–28Every 4 weeks
Weeks 28–36Every 2 weeks
Weeks 36–40Weekly
Week 40 and beyond1–2 times per week, often including non-stress tests and fluid checks

By week 28

Glucose screening, Rh factor follow-up if needed, start researching pediatricians and maternity leave paperwork.

By week 36

Pack your hospital bag, install and inspect your car seat, pre-register at your delivery hospital, finalize your birth plan.

After your due date

Attend any added monitoring appointments, discuss your provider’s specific induction timing policy, keep your bag by the door.

Putting together your hospital bag is one of the easiest things to get ahead of — see the interactive Hospital Bag Checklist for a category-by-category, printable packing list.

Related Reading

Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.

Frequently asked questions

Rarely. Once it’s confirmed at a first-trimester scan, ACOG guidance says it should generally stay fixed for the rest of the pregnancy, even if later scans estimate your baby as measuring slightly larger or smaller — those later measurements monitor growth, not your dates.
Because it’s calculated as 280 days (40 weeks), which works out to about nine months and one week — a result of counting in 7-day weeks rather than calendar months of uneven length.
Yes. EDD (estimated due date) and EDC (estimated date of confinement, an older clinical term) both refer to the same calculated date.
On average, slightly — research has found later pregnancies run a few days shorter than first pregnancies. The difference is small, though, and not reliable enough to predict any individual birth.
The calculation method is the same, but the typical delivery window shifts earlier. Most twin pregnancies are considered full term around 37–38 weeks rather than 39–40, since multiple pregnancies statistically run shorter on average.
Trust your provider’s number. Most apps use a basic LMP-plus-280-days calculation without factoring in your specific cycle length or ultrasound confirmation, both of which your provider takes into account.
No — going past the due date itself is common and expected for a large share of pregnancies. Most providers begin closer monitoring around 41 weeks rather than treating week 40 as a deadline.
From the exact embryo transfer date, since the embryo’s age is precisely known at transfer — a method considered more accurate than LMP-based dating used for naturally conceived pregnancies.
Medically reviewed by Dr. Ruqaiya Khan, MD, FACOG
Board-Certified Obstetrician–Gynecologist

Dr. Khan reviews all dating, term-pregnancy, and gestational-age content on PregCalc.com for clinical accuracy against current ACOG, NHS, and Mayo Clinic guidance. Last reviewed: . View reviewer credentials →

Sources: American College of Obstetricians and Gynecologists (ACOG) — Definition of Term Pregnancy, Committee Opinion No. 579 · ACOG Practice Bulletin No. 146 — Management of Late-Term and Postterm Pregnancies · NHS — Pregnancy Due Date Guidance · Mayo Clinic · NICHD

Plan, track and prepare with our other medically reviewed tools.

Due Date Calculator

Estimate your due date from your last period, cycle length or conception.

Open calculator

Ovulation Calculator

Find your most fertile days and predicted ovulation date.

Open calculator

Gestational Age Calculator

Check exactly how many weeks and days pregnant you are.

Open calculator

Conception Calculator

Work back from your due date to estimate when you conceived.

Open calculator