How Due Dates Are Calculated — And Why Most Babies Don't Arrive on Them

Most babies don't arrive on their due date — and that's completely normal

The due date is the most anticipated single day of pregnancy. It goes on the calendar. Family travel plans are arranged around it. And then, more often than not, the baby arrives on a different day entirely. Only about 4–5% of babies are born on their calculated due date. More than half arrive within a week of it, and most first-time births happen slightly later.

Understanding how due dates are calculated — and how much natural variation surrounds them — helps set realistic expectations and avoids the anxiety that builds when the date passes without the baby arriving.

How the due date is calculated: Naegele's Rule

The standard due date formula is Naegele's Rule, developed in the early 19th century by German obstetrician Franz Karl Naegele:

Due date = First day of last menstrual period (LMP) + 280 days (40 weeks)

Or equivalently: LMP + 9 months + 7 days.

For example, if the LMP was March 1: March 1 + 280 days = November 6 as the estimated due date.

The formula assumes:

• A standard 28-day menstrual cycle
• Ovulation occurring on day 14 of the cycle
• Fertilisation occurring at ovulation

Actual conception occurs 2 weeks into the 40-week count — meaning the "40 weeks of pregnancy" includes approximately 2 weeks before conception took place. This is why gestational age is counted from LMP rather than from conception: the exact date of ovulation and conception is rarely known with certainty.

Why the LMP method is imprecise

Naegele's Rule is a reasonable approximation, but several factors introduce error:

Cycle length variation. The formula assumes a 28-day cycle. Many women have cycles of 24–35+ days. A woman with a 35-day cycle ovulates around day 21 rather than day 14, meaning conception — and therefore gestational age — is systematically miscalculated if LMP is used without adjustment. The resulting due date would be about a week earlier than it should be.

Irregular cycles. Cycles affected by stress, recent hormonal contraceptive use, breastfeeding, or health conditions may not follow predictable patterns at all. LMP-based calculation is less reliable in these cases.

Uncertain LMP. Not all people reliably remember the first day of their last period with confidence. A date range of ±5 days introduces corresponding uncertainty in the due date.

Ultrasound dating: more accurate after LMP

A first-trimester ultrasound (typically between 8–14 weeks) measures the crown-rump length (CRL) of the embryo. Growth at this early stage is consistent enough across pregnancies that CRL is a reliable indicator of gestational age — accurate to approximately ±5–7 days.

When ultrasound dating takes precedence over LMP:

• If there's a discrepancy of more than 5–7 days between the ultrasound estimate and the LMP-based estimate, the ultrasound date is used
• When the person has irregular cycles
• When the LMP date is uncertain

After the first trimester, growth variation between fetuses increases, making ultrasound measurement less reliable for dating — which is why early ultrasound provides the most accurate gestational age estimate.

The actual distribution of birth timing

The due date is an estimate of the statistical centre of a distribution, not a prediction of the specific day. The actual distribution of birth timing at term looks approximately like this:

• Before 37 weeks: preterm birth (~10% of births globally; varies by population)
• 37–38 weeks: early term
• 39–40 weeks: full term (peak of the distribution)
• 41 weeks: late term
• 42+ weeks: post-term

The distribution peaks slightly past 40 weeks for first-time births (primigravida pregnancies). Studies of spontaneous labour onset find the median is closer to 40 weeks + 5 days for first pregnancies and 40 weeks + 3 days for subsequent pregnancies.

This means waiting 1–2 weeks past the calculated due date without delivering is entirely within normal range for most pregnancies.

What "term" means

"Term" has evolved in clinical practice to a more nuanced set of definitions than the single "40 weeks" figure:

• Early term: 37 weeks 0 days – 38 weeks 6 days
• Full term: 39 weeks 0 days – 40 weeks 6 days
• Late term: 41 weeks 0 days – 41 weeks 6 days
• Post-term: 42 weeks 0 days and beyond

This distinction matters clinically because outcomes for babies born at 37–38 weeks are meaningfully different from those at 39–40 weeks — early term babies have higher rates of respiratory complications, NICU admission, and breastfeeding difficulties than full-term babies. Elective procedures before 39 weeks are increasingly discouraged unless medically indicated.

Post-dates pregnancy: what happens after 40 weeks

Pregnancies that continue past 42 weeks are defined as post-term. The risk profile changes at this point — placental function declines, amniotic fluid volume may decrease, and the risk of stillbirth and meconium aspiration increases slightly compared to full-term births.

Most obstetric guidelines recommend discussing induction of labour at 41 weeks, and some recommend offering it at 40 weeks +10 days to 40 weeks +14 days, based on evidence that induction at this point reduces risk without increasing caesarean rates (based on the ARRIVE Trial and subsequent meta-analyses).

The specific policy varies by country and healthcare provider. NICE guidelines in the UK recommend offering induction from 41 weeks onwards. The American College of Obstetricians and Gynecologists (ACOG) supports elective induction from 39 weeks.

How to use the Due Date Calculator on sadiqbd.com

From LMP:

1. Enter the first day of your last menstrual period
2. Optionally adjust for cycle length if it differs from 28 days
3. Read the estimated due date and gestational milestones

From conception date: If you know the approximate date of conception (via ovulation tracking, IVF transfer date, or confirmed ovulation), entering the conception date gives a more accurate due date than LMP-based calculation.

From ultrasound date: Enter a confirmed gestational age from an ultrasound report to calculate the corresponding due date.

The calculator also shows approximate milestone weeks — when first trimester ends, when anatomy scan is typically performed, third trimester start, and various gestational age benchmarks.

Frequently Asked Questions

Why is pregnancy counted from the last period rather than conception? Because the exact date of conception is rarely known with certainty, while the first day of the last period is more reliably remembered. The 40-week count from LMP includes approximately 2 weeks before conception occurred. LMP-based dating is a practical convention, not a biological statement about when pregnancy "started."

What happens if my due date changes after an ultrasound? It's common. If the first-trimester ultrasound measurement suggests a different gestational age than the LMP calculation, the due date is revised to match the ultrasound. This doesn't mean the pregnancy is abnormal — it usually reflects cycle length variation or LMP uncertainty.

Is it safe to go past 40 weeks? Yes, within limits. Late-term (41 weeks) and early post-term (42 weeks) are within the normal range for spontaneous labour, and most babies born in this period are completely healthy. Monitoring increases after 41 weeks (non-stress tests, biophysical profiles) and most guidelines recommend discussing induction after 41 weeks.

Is the Due Date Calculator free? Yes — completely free, no sign-up required.

A due date is a reasonable central estimate around a distribution that spans several weeks. Understanding the natural variation — and the way ultrasound, cycle length, and LMP interact in the calculation — makes it easier to approach the final weeks of pregnancy with accurate expectations rather than unnecessary anxiety.

Try the Due Date Calculator free at sadiqbd.com — find your estimated due date from LMP, conception date, or ultrasound, with gestational milestones included.