Most people assume a pregnancy lasts nine months. Technically it is closer to ten lunar months — about 40 weeks, or 280 days, counted from the first day of the last menstrual period. That 280-day figure is the heart of Naegele's rule, the clinical method used worldwide since the 1830s. Understanding how it works — and why your confirmed due date from an ultrasound may differ from the LMP estimate — makes the whole first trimester a lot less confusing.
The Naegele Rule — 280 Days From LMP
Franz Karl Naegele, a German obstetrician, formalised the rule in 1830: add 280 days (40 weeks) to the first day of your last menstrual period. A quick alternative for a 28-day cycle is to add 9 months and 7 days to the LMP. A large 2013 study published in Human Reproduction (Jukic et al.) confirmed an average gestation of 280 days from LMP, though genuine variation of ±2 weeks is completely normal — only about 5% of babies actually arrive on their predicted due date.
EDD (28-day cycle) = LMP + 280 days Cycle adjustment: EDD = LMP + 280 + (Your cycle length − 28) days Example (32-day cycle): LMP: 1 January 2026 Adjustment: +4 days (32 − 28) EDD: 1 January + 280 + 4 = 11 October 2026 Conception date method: EDD = Conception date + 266 days (38 weeks from fertilisation)
Why Cycle Length Changes Your EDD
Naegele's rule assumes ovulation on day 14 of a 28-day cycle. If your cycle runs longer — say, 35 days — you likely ovulate around day 21, meaning conception happened about a week later than the standard formula assumes. The EDD is therefore pushed forward by the same number of days. This adjustment is small but meaningful: a 7-day error in the EDD can affect clinical decisions around the timing of an induction.
How Ultrasound Can Override the LMP Date
A first-trimester ultrasound, performed between 8 and 13 weeks 6 days gestation, measures the crown-rump length (CRL) of the embryo. This measurement is highly accurate at predicting gestational age — the margin of error is roughly ±5 days before 9 weeks and ±7 days at 13 weeks. ACOG guidelines recommend revising the EDD to the ultrasound estimate if it differs from the LMP-based date by more than 5–7 days in the first trimester. This is why many women leave their first scan with a different due date from what an online calculator showed them.
The Three Trimesters — What Each Means
First trimester: Weeks 1–12 • Major organ formation (organogenesis) • Highest miscarriage risk (most fall in weeks 6–10) • Key scan: Nuchal translucency + dating scan at 11–13+6 weeks Second trimester: Weeks 13–27 • Reduced miscarriage risk; baby begins to move • Key scan: Anatomy (anomaly) scan at 18–22 weeks • AFP/quad screen: typically weeks 15–20 Third trimester: Weeks 28–40+ • Rapid growth; lungs mature • GBS swab: week 36 • Early term: 37–38 weeks; full term: 39–40 weeks • Late term: 41 weeks; post-term: 42+ weeks (monitoring intensifies)
Tip
Keep a printed or digital record of your LMP date and your first-scan date. When switching providers or moving to a new hospital, both dates help the new team quickly reconstruct your pregnancy timeline without relying on the history you give verbally under stress.
When a Due Date Is 'Confirmed'
Clinically, a due date is not truly confirmed until a first-trimester ultrasound is documented in your antenatal record. The LMP-based estimate is used as a working date until that scan. After week 20, ultrasound estimates become less accurate because babies grow at different rates, and the EDD is generally not revised based on a late-pregnancy scan alone unless there is a documented reason.