Pregnancy Due Date Calculator

Calculating your estimated due date is one of the first milestones of confirmed pregnancy. The number your doctor or midwife gives you is not a guess — it is derived from a standardised formula that has been used in obstetrics for nearly two centuries. Understanding how that number is calculated, why it is only an estimate, and what factors can shift it helps set realistic expectations for one of the most anticipated dates in a person's life.

Naegele's Rule: The Standard Formula

Naegele's rule, formulated by German obstetrician Franz Karl Naegele in 1812, remains the foundational method for calculating due dates worldwide. The rule adds 280 days (40 weeks) to the first day of the last menstrual period. In practice, this is often calculated by adding one year, subtracting three months, and adding seven days to the LMP date. A woman whose last period began on January 1st would have an estimated due date of October 8th of the same year. The 280-day figure assumes a standard 28-day menstrual cycle with ovulation on day 14.

Naegele's original formula was developed before the mechanisms of fertilisation and implantation were understood, yet it remains remarkably accurate as a population-level estimate. Modern obstetric practice supplements it with ultrasound measurements rather than replacing it, since the two methods together provide better accuracy than either alone.

Adjusting for Cycle Length

The standard Naegele formula assumes a 28-day cycle. Women with longer cycles tend to ovulate later than day 14, meaning conception occurs later — and the due date should be adjusted forward accordingly. Women with shorter cycles ovulate earlier, shifting the due date back. Our calculator automatically accounts for this: for each day your average cycle length differs from 28, the due date is shifted by the same amount. A woman with a consistent 35-day cycle would have her due date moved seven days later compared to the standard calculation.

This adjustment improves accuracy for women with irregular or atypical cycles, but it still relies on the assumption that your cycle is consistent and that ovulation occurs 14 days before the next expected period. If your cycles are highly variable, early ultrasound dating is likely to provide a more reliable due date estimate than any formula-based calculation.

The Role of First-Trimester Ultrasound

First-trimester ultrasound, typically performed between 8 and 13 weeks, measures the crown-rump length (CRL) of the embryo. At this stage of development, fetal size is highly consistent between individuals — differences in genetics and maternal factors have not yet had time to create significant size variation. This makes CRL measurement the most accurate non-invasive method for dating a pregnancy. If the ultrasound-derived gestational age differs from the LMP calculation by more than 5–7 days, most guidelines recommend revising the due date to match the ultrasound.

After 13 weeks, fetal biometry measurements become less accurate for dating purposes because individual genetic variation in growth rates starts to play a larger role. The due date established by first-trimester ultrasound or confirmed LMP remains the reference point throughout the entire pregnancy, even as later scans show growth that may be ahead of or behind the average.

What Happens Before and After the Due Date?

A pregnancy is considered full-term from 39 weeks to 40 weeks and 6 days. Early term is 37 to 38 weeks and 6 days, and late term is 41 weeks to 41 weeks and 6 days. Post-term is 42 weeks or beyond. Babies born in the early term window may face slightly higher risks of respiratory and feeding difficulties compared to those born at full term, even though they are not premature by traditional definitions. Most obstetric units will recommend induction of labour by 41–42 weeks to reduce the risks associated with post-term pregnancy, including placental insufficiency.

Only about 4% of babies are born on their exact due date. The normal range for spontaneous labour spans roughly three to four weeks around the due date. The calculated due date is best understood as the midpoint of a window rather than a deadline — a planning tool rather than a precise prediction.