How Due Dates Are Calculated (And Why They Change)

Why 40 Weeks? The Basis of Due Date Calculation

A full-term pregnancy is conventionally defined as 40 weeks (280 days) measured from the first day of the last menstrual period (LMP) — not from the date of conception. This convention exists because historically the LMP date was the earliest reliably known date related to pregnancy.

The 40-week convention counts 2 weeks of the menstrual cycle before ovulation and conception actually occurs (for a standard 28-day cycle). This means gestational age and fetal age are different:

• Gestational age is counted from LMP — this is what your provider uses
• Fetal age (embryonic age) is counted from fertilization — approximately 2 weeks less than gestational age for a 28-day cycle

So when a provider says "you are 8 weeks pregnant," they mean 8 weeks from your LMP — the fetus itself is approximately 6 weeks old.

Naegele's Rule: LMP-Based Dating

The most common method for estimating the due date from LMP is Naegele's rule, developed in the 19th century and still widely used:

EDD = LMP + 280 days

Alternatively: take the first day of your LMP, add 1 year, subtract 3 months, add 7 days.

Example: LMP = January 15, 2026 → EDD = October 22, 2026 (January + 9 months + 7 days)

Naegele's rule has two built-in assumptions:

1. A 28-day menstrual cycle
2. Ovulation occurs on day 14 of that cycle

For women with longer or shorter cycles, the EDD must be adjusted. For a 30-day cycle (ovulation on day 16, 2 days later than assumed), add 2 days to the EDD. For a 25-day cycle (ovulation on day 11), subtract 3 days. Our due date calculator handles these adjustments automatically.

Why Cycle Length Matters for Dating Accuracy

The standard Naegele EDD is reasonably accurate for women with 28-day cycles, but systematically off for those with longer or shorter cycles. A woman with a 35-day cycle ovulates approximately 7 days later than the 28-day assumption, meaning her actual conception was 7 days later — and her EDD is 7 days later than Naegele would suggest.

This is a real clinical issue: if the LMP-based EDD is used for a woman with a 35-day cycle and an early ultrasound is not done, the baby may appear "small" or "behind" when in fact dating is simply off by a week. Early ultrasound dating (before 14 weeks) catches and corrects these discrepancies.

Ultrasound Dating: The Gold Standard

First-trimester ultrasound (performed between 8 and 13+6 weeks gestation) is the most accurate method for establishing gestational age and EDD. It measures crown-rump length (CRL) — the length of the fetus from the top of the head to the bottom, with the fetus in a neutral position.

CRL measurement accuracy:

• 8-10 weeks: ±5 days (the most accurate window)
• 10-13 weeks: ±5-7 days
• After 14 weeks: CRL becomes less reliable as the fetus curls — biometric measurements are used instead

Second-trimester ultrasound (14-20 weeks) uses multiple measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). These have an accuracy of approximately ±10-14 days, which is why the second-trimester anatomy scan is less precise for establishing EDD than first-trimester CRL measurement.

ACOG Guidelines on When to Revise the EDD

ACOG (American College of Obstetricians and Gynecologists) Committee Opinion #700 (2017) provides guidance on when to revise the EDD based on ultrasound findings:

• Before 9 weeks: Revise if CRL-based EDD differs by more than 5 days from LMP-based EDD
• 9-13+6 weeks: Revise if CRL-based EDD differs by more than 7 days from LMP-based EDD
• 14-15+6 weeks: Revise if ultrasound EDD differs by more than 7 days
• 16-21+6 weeks: Revise if ultrasound EDD differs by more than 10 days
• After 22 weeks: Do not revise EDD based on late-pregnancy ultrasound (too inaccurate for dating at this stage)

Once established with an early ultrasound, the EDD should generally remain consistent throughout pregnancy — it is not revised every time a growth scan shows the baby measuring differently. Growth scans assess growth trajectory, not gestational age.

IVF Dating: More Precise Than LMP

For pregnancies conceived through IVF, the exact date of fertilization is known — making IVF dating more precise than LMP-based calculations. The formula depends on the embryo's age at transfer:

• Day 5 blastocyst transfer: EDD = Transfer date + 261 days (266 days from fertilization minus 5 days of embryo age)
• Day 3 embryo transfer: EDD = Transfer date + 263 days (266 minus 3)
• Day 6 blastocyst transfer: EDD = Transfer date + 260 days (266 minus 6)

Some providers express this differently using the "LMP equivalent" — a hypothetical LMP date that would produce the same EDD using Naegele's rule. This allows IVF pregnancies to be tracked in the same gestational age framework as LMP-based pregnancies.

Only 4-5% of Babies Are Born On Their Due Date

The term "due date" implies more precision than the science supports. Research shows:

• Only about 4-5% of births occur on the exact EDD
• Approximately 80% of births occur between 37-41 weeks
• First-time mothers give birth on average 1-3 days after their EDD
• Subsequent pregnancies tend to deliver slightly closer to the EDD or slightly earlier

ACOG defines "full term" as 39-40 weeks, "early term" as 37-38 weeks, and "late term" as 41 weeks. The EDD is best understood as the center of a 5-week window (37-42 weeks) rather than a specific predicted date. Expecting your baby in "October" rather than "on October 15" leads to a more relaxed approach to the last weeks of pregnancy.

Practical Advice for Your Appointment

To get the most accurate dating at your first prenatal appointment:

• Know the exact first day of your last menstrual period
• Be ready to describe your typical cycle length (not just "28 days" if that is not accurate for you)
• Mention if you used ovulation tracking, OPKs, or IVF — providers can use this data to improve dating accuracy
• Request an early ultrasound (before 14 weeks) if your cycles are irregular, you have a poor memory of your LMP, or you conceived with fertility treatments