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How Accurate Is a Pregnancy Due Date? The Math Behind Naegele's Rule
Only about 4% of babies arrive on the predicted day. Here is why — and how clinicians refine the estimate.
The first thing almost every newly-pregnant person asks is, "When is the baby due?" Their clinician will write down a single calendar date. That date will be repeated by family, employers, baby-shower invitations, and hospital schedulers as if it were a guarantee. It is not. It is the centre of a probability distribution, and most of that probability lives in the two weeks on either side. This article explains where the due date comes from, how reliable it is, and what to do when the calendar and biology disagree.
This article is an educational explainer, not medical advice. Pregnancy dating decisions should always be made with a qualified healthcare professional.
Where the due date comes from
The standard formula for estimating a pregnancy due date is Naegele\u2019s rule, named after the early-19th-century German obstetrician Franz Karl Naegele who codified it. The rule is:
Estimated due date = first day of last menstrual period + 280 days
Equivalently: take the first day of your last period, subtract three months, add seven days, and add one year. Naegele assumed a regular 28-day menstrual cycle with ovulation on day 14, which means conception occurs roughly two weeks after the start of the last period. The rule produces a 40-week pregnancy from the first day of the last menstrual period (LMP), or a 38-week pregnancy from conception.
Why "last menstrual period" rather than "conception"?
Most people know the date of their last period. Far fewer know the date of conception. LMP is therefore a more practical reference point for routine clinical use, even though it is roughly two weeks earlier than the biologically meaningful event.
This is why doctors describe pregnancy in "gestational weeks" rather than "weeks since conception". Gestational age 12 weeks means twelve weeks since the LMP, which is approximately ten weeks of actual fetal development. Most pregnancy textbooks, parenting apps, and ultrasound reports use gestational age. If you are tracking the embryo\u2019s development, subtract two weeks; if you are tracking your pregnancy administratively, do not.
How accurate is the prediction?
About 4% of babies are born on their predicted due date. Roughly 50% are born within one week of the date in either direction; about 80% within two weeks. The 2013 study by Jukic et al., which tracked early-pregnancy hormones in carefully-monitored volunteers, found that the natural variation in pregnancy length is around 37 days — much wider than most people realize.
Several factors affect the spread:
- Cycle length. Naegele\u2019s rule assumes a 28-day cycle. People with longer cycles tend to deliver later than predicted; people with shorter cycles tend to deliver earlier. The simple correction is to add or subtract the difference in cycle length from the standard 28 days.
- First pregnancy vs. subsequent. First-time mothers tend to deliver about five days later than the predicted date, on average. Mothers in subsequent pregnancies tend to deliver closer to the date, or slightly earlier.
- Ethnicity and family history. Studies show small but real variations in average pregnancy length across populations, on the order of a few days.
- Maternal age and BMI. Older mothers and mothers with higher BMI tend to have slightly longer pregnancies on average.
Why ultrasound dating is more accurate
Crown-rump length (CRL) measured by ultrasound between 7 and 13 weeks of gestation is the most accurate way to date a pregnancy. The relationship between CRL and gestational age is tight in the first trimester — most fetuses develop at the same rate during this period, regardless of the mother\u2019s underlying biology — and the ultrasound estimate is typically accurate to within five to seven days.
If the LMP-derived date and the first-trimester ultrasound disagree by more than seven days, current obstetric guidelines (ACOG / SMFM) recommend updating the estimated due date to match the ultrasound. Later in pregnancy the ultrasound becomes less accurate at dating, because individual fetal growth varies more.
What "due date" really means clinically
The due date defines the centre of the term-pregnancy window. Pregnancies are now classified as:
- Preterm: before 37 completed weeks of gestation.
- Early term: 37 weeks 0 days through 38 weeks 6 days.
- Full term: 39 weeks 0 days through 40 weeks 6 days. This is when the average baby is best off being born.
- Late term: 41 weeks 0 days through 41 weeks 6 days.
- Postterm: 42 weeks 0 days and beyond. Most clinicians recommend induction by this point because risks to the baby start to rise.
The due date is therefore most useful as a reference point for these clinical windows, not as a calendar reservation. A baby born five days before or five days after the due date is squarely "on time".
Common myths
Myth: A full moon makes labor more likely on the due date. Multiple large cohort studies have looked for a lunar effect and found nothing. Labor onset is not influenced by lunar phase.
Myth: Lots of walking will make the baby come on time. Walking is excellent for general health and may help with some pregnancy discomforts, but it does not measurably accelerate the onset of labor.
Myth: Spicy food induces labor. No clinical evidence supports this, though it is a beloved tradition in many cultures.
Myth: First babies are always late. First babies are, on average, late by about five days. But "average" hides huge variation — many first babies arrive early, and many later babies arrive even later than their older siblings did.
What if I don\u2019t remember my LMP?
Many people do not remember the exact date of their last menstrual period — perhaps because their cycles were irregular, perhaps because they were not tracking. In that case, your clinician will rely on a first-trimester ultrasound for dating. This is now standard practice and produces a very reliable estimate.
What if my cycles are irregular?
Naegele\u2019s rule assumes a regular 28-day cycle. If your cycles are irregular or substantially longer than 28 days, the LMP-derived date will be less reliable. An early ultrasound is essentially mandatory in this case to get an accurate due date.
What can be done with the due date?
Apart from anchoring routine prenatal-care milestones (first trimester screening, anatomy scan, glucose tolerance test, third-trimester monitoring), the due date is the reference point for:
- Maternity-leave planning — many statutory leave schemes start a fixed number of weeks before the due date.
- Insurance benefit calculations for parental leave and short-term disability.
- Decisions about induction if the pregnancy reaches the late-term or postterm window.
- School year placement of the future child — in most countries, school-year cut-offs are based on calendar age at the start of the academic year. A child born even a day before the cut-off can end up in a different grade than a child born a day later.
How our pregnancy due date calculator handles the numbers
Our pregnancy due date calculator implements the standard Naegele formula with the optional cycle-length correction described above. It also reports your current gestational age (weeks plus days), trimester, and remaining time. As with every tool on this site, the calculation runs entirely in your browser; we never see your last menstrual period or any other input.
Use it to set expectations and to double-check your clinician\u2019s figure — but for any decision that depends on accurate dating, an early ultrasound performed by a qualified professional is the gold standard.
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