You take a positive pregnancy test, tell your doctor, and get a due date. Where does that number come from? It is not the date you actually conceived — it is calculated from your last menstrual period using a 200-year-old formula, and it has some quirks that are worth understanding.
The classic method: Naegele’s rule
Most due dates are calculated using Naegele’s rule, developed by German obstetrician Franz Naegele in the early 1800s. The formula:
Due date = First day of last menstrual period (LMP) + 280 days
Or equivalently:
Due date = First day of LMP + 1 year − 3 months + 7 days
Example: LMP started January 1. Due date = January 1 + 1 year − 3 months + 7 days = October 8.
The 280 days equals 40 weeks. This is the standard pregnancy length counted from the first day of the last period — not from conception.
Why 40 weeks from LMP?
Pregnancy is typically measured from the first day of the last menstrual period because that date is relatively easy to remember and document. Conception itself happens about two weeks later, around ovulation in a typical 28-day cycle.
So when you are “4 weeks pregnant,” you actually conceived about 2 weeks ago. When you are “40 weeks pregnant,” your fetus has been developing for about 38 weeks. The total fetal development time is about 38 weeks (266 days); the pregnancy count just starts earlier for convenience.
This is confusing to most first-time parents. A pregnancy test says positive around week 4, meaning you have been pregnant for about 2 weeks by the biological clock but 4 weeks by the obstetric clock. Both numbers are “correct” — they refer to different start points.
Assumptions behind Naegele’s rule
The rule assumes:
- A 28-day menstrual cycle
- Ovulation on day 14
- Conception within 24 hours of ovulation
- A “normal” pregnancy length of 280 days from LMP
Reality is messier. Normal cycles range from 21 to 35 days. Ovulation timing varies. Not everyone ovulates every cycle. Pregnancy length itself varies: only about 5% of babies are born on their calculated due date. The normal range is 37-42 weeks.
Ultrasound dating (more accurate)
If your cycle is irregular, you cannot remember your LMP, or you are unsure, ultrasound dating during the first trimester provides a more accurate due date. Early ultrasound measures the embryo or fetus size — especially the crown-rump length — and compares it to standardized growth curves to estimate gestational age.
First-trimester ultrasound (before 13 weeks) is accurate to within about ±5-7 days. Second-trimester ultrasound accuracy drops to ±10-14 days. Third-trimester dating is not reliable for setting a due date because fetal growth variability is too high by then.
If ultrasound dating differs from LMP dating by more than 7 days in the first trimester, most obstetricians redate the pregnancy to the ultrasound-based due date. This is especially common with longer cycles (32+ days), recent hormonal contraceptive use, or irregular ovulation.
Conception date vs LMP
If you know your conception date (for example, from IVF or a single intercourse event during ovulation):
Due date = conception date + 266 days (38 weeks)
This is equivalent to LMP + 280 days if you ovulated on day 14 of a 28-day cycle. If your cycle is longer or shorter, this formula gives a different (more accurate) due date than Naegele’s rule.
What the due date actually predicts
Your due date is an estimate of when you have a 50-50 chance of having given birth. Statistically:
- 3-5% of babies are born on the exact due date.
- About 50% are born within 7 days of the due date.
- About 80% are born within 14 days.
- First pregnancies average about 8 days past the due date.
- Subsequent pregnancies average about 3 days past the due date.
So the due date is a reference point, not a prediction. Think of it as the center of a bell curve. The most likely 2-week window is from 39 weeks to 41 weeks, with spillover on both sides.
Terms you will encounter
- Full-term: 37-42 weeks. Modern obstetrics further divides this:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
- Preterm: Before 37 weeks. Further subdivided into extremely preterm (under 28), very preterm (28-32), moderate preterm (32-34), and late preterm (34-37).
- Gestational age: Weeks from LMP. How obstetrics counts.
- Embryonic age or fetal age: Weeks from conception. Approximately LMP − 2 weeks.
- Trimesters: First trimester ends at about 13-14 weeks. Second ends at about 27-28 weeks. Third runs to delivery.
When due dates get revised
Doctors may revise a due date when:
- First-trimester ultrasound differs from LMP-based date by more than 5-7 days
- You are uncertain about your LMP
- Your cycles are highly irregular
- IVF was used (conception date is known precisely)
Once the first trimester ends, doctors generally do not revise the due date based on later ultrasounds. The goal is a stable reference point for the rest of the pregnancy, not continual adjustment.
What the due date is used for
- Scheduling the prenatal care timeline (first trimester screening, anatomy scan, glucose test, group B strep, induction planning)
- Tracking fetal development against standard milestones
- Deciding when preterm labor is a concern (before 37 weeks) vs a normal early delivery
- Triggering induction discussions if pregnancy extends past 41-42 weeks
- Planning parental leave and childcare arrangements
- Determining viability and intervention thresholds
Factors that do NOT shift the due date
- Size of the baby on ultrasound later in pregnancy (babies grow at different rates; size at 30 weeks does not redate)
- When you felt movement (varies widely, 16-24 weeks)
- When the pregnancy test turned positive
- The mother’s physical symptoms
Calculate yours
Our pregnancy due date calculator uses Naegele’s rule and also allows a conception-date input if you know it. Plug in the first day of your last period or the known conception date, and the tool returns your estimated due date plus gestational age for today. Useful for tracking milestones, planning appointments, and answering the question you will be asked constantly during pregnancy: “When are you due?”
Just remember the due date is a target, not a promise. Most babies arrive within two weeks either way.