Estimated Due Date

Estimated due date: Nägele's rule

Nägele's rule works out the EDD (estimated due date) from the LMP (last menstrual period): EDD = LMP + 7 days − 3 months + 1 year. The math assumes a regular 28-day cycle with ovulation on day 14. A full pregnancy runs about 280 days, or 40 weeks counted from the LMP (38 weeks from conception). Take an example: an LMP on Jan 1, 2026 gives an EDD of Oct 8, 2026. If the cycle is longer or shorter, just add (cycle length − 28) days. The real benchmark, though, is the first-trimester ultrasound (up to 13 weeks 6 days). It is the gold standard for dating and overrides the LMP-based EDD whenever the gap goes past 5–7 days.

Clinical context

The Brazilian Society of Pediatrics (SBP) splits things this way: early term 37 weeks to 38w6d, full term 39w to 40w6d, late term 41 weeks, post-term ≥42 weeks. Only about 4% of babies actually arrive on the exact EDD, while roughly 80% show up between 37 and 42 weeks. This date feeds into a lot of things: SUS prenatal care (the HumanizaSUS handbook), pregnancy work-leave certificates, maternity and paternity planning under Brazilian Law 13.257/2016 (which extended leave), and the timing of exams such as the morphological ultrasound and GBS screening at 35–37 weeks.

FAQ

What if my cycle isn't 28 days? Add or subtract the difference. A 32-day cycle moves the EDD 4 days later, and a 25-day cycle pulls it 3 days earlier. Even so, dating by ultrasound is the better bet.

How accurate is the EDD? Figure on ±2 weeks. A first-trimester ultrasound narrows that to ±5–7 days, and the later the scan, the less you can trust it for dating.

Can I calculate from conception date instead? Sure. Add 266 days (38 weeks) to the conception date. This helps with IVF or any time you know the ovulation date.

What if I don't remember my LMP? Go with a first-trimester ultrasound, which is the safest route. A few calculators try to estimate from fundal height, but that is far less accurate.