Pregnancy Due Date Calculator

Naegele's rule — a brief history

The standard method for calculating a due date is Naegele's rule, named after German obstetrician Franz Karl Naegele, who described it in 1812. The rule is straightforward: add 280 days (40 weeks) to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation occurring on day 14.

Despite being over 200 years old, Naegele's rule remains the standard starting point for estimating due dates worldwide. Modern obstetric practice combines it with first-trimester ultrasound for more precise dating — ultrasound measures the crown-rump length of the fetus, which correlates strongly with gestational age in early pregnancy.

One limitation of the original rule: it assumes a 28-day cycle. Women with longer cycles (for example, 35 days) tend to ovulate later — around day 21 rather than day 14 — which means their due date should be pushed forward by approximately 7 days. This calculator applies this adjustment when you specify a non-standard cycle length.

How due dates are used clinically

The estimated due date (EDD) is used clinically as a reference point for:

• •Scheduling prenatal care. Appointments, screenings, and tests are timed relative to gestational age — e.g., the 20-week anatomy scan, glucose tolerance test at 24–28 weeks, Group B Strep test at 35–37 weeks.
• •Determining full term. A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42 weeks or beyond.
• •Timing interventions. Post-term pregnancies (42+ weeks) carry increased risk and may prompt induction discussions. Preterm labour is defined as before 37 weeks.
• •Interpreting test results. Many prenatal screening results (e.g., AFP, nuchal translucency) are interpreted in relation to gestational age.

What affects accuracy

Only about 5% of babies are born on their exact due date. The distribution of birth dates around the EDD is wide — roughly 80% of births occur between 37 and 42 weeks. Several factors affect how close the EDD is to actual birth:

• •Cycle regularity. Irregular cycles make LMP-based dating less reliable. The more variable your cycle, the larger the uncertainty in your EDD.
• •Ultrasound dating. A first-trimester ultrasound (before 14 weeks) can date a pregnancy to within 5–7 days. Second-trimester scans have wider margins of error. Clinical practice often revises the EDD based on ultrasound if it differs from LMP-based dating by more than 5–10 days.
• •First vs. subsequent pregnancies. First-time mothers (nulliparous women) tend to go slightly longer — statistical analysis suggests average delivery at 41 weeks 1 day for first pregnancies, versus 40 weeks 3 days for subsequent pregnancies.
• •Genetics. There is a heritable component to gestational length. Women whose mothers had longer or shorter pregnancies tend to have similar patterns.

Trimester milestones

The three trimesters each have distinct developmental milestones and common experiences:

• •First trimester (weeks 1–12): All major organ systems form. The neural tube closes by week 6. The heart begins beating around week 6. By week 12, the fetus has identifiable facial features and limb buds. Nausea, fatigue, and breast tenderness are common. Miscarriage risk is highest in this period.
• •Second trimester (weeks 13–26): Often called the “honeymoon trimester” — nausea typically resolves, energy improves. The 20-week anatomy scan checks fetal development and can reveal sex. The fetus develops hearing and begins responding to sound. By week 24, the fetus reaches the threshold of viability outside the womb (with intensive medical support).
• •Third trimester (weeks 27–40+): Rapid weight gain for both mother and baby. The lungs mature (surfactant production complete by ~36 weeks). The fetus positions for birth, typically head-down by 36 weeks. Braxton Hicks contractions increase. Fatigue, back pain, and frequent urination are common.

Frequently asked questions