Due Date & Pregnancy Milestones
A gentle estimate of your due date and the key milestones ahead, based on your last menstrual period.
How this estimate is derived
- Model:
- Naegele's rule (last menstrual period + 280 days)
- Implementation last reviewed:
- 2026-08-29
- Purpose:
- Educational estimate only; not a substitute for clinical dating by ultrasound.
How this is calculated
This uses Naegele's rule — adding 280 days (40 weeks) to the first day of your last menstrual period — the standard, widely used starting estimate for a due date. Milestone weeks reflect common timing for routine scans and pregnancy stages, though your own care provider's schedule may vary slightly.
It's worth setting expectations around this figure: only around 4% of babies are actually born on their estimated due date. The normal range for a healthy, full-term birth spans roughly 37 to 42 weeks, so the due date is best treated as the midpoint of a several-week window rather than a firm deadline. Once your baby arrives, the Sleep Cycle calculator can help with working out your own optimal bedtimes around a newborn's unpredictable schedule.
Frequently asked questions
- Why is due date calculated from my last period, not conception?
- Because conception date is rarely known precisely, but the start of the last menstrual period usually is. Naegele's rule (LMP + 280 days, i.e. 40 weeks) assumes a fairly typical 28-day cycle with ovulation around day 14 — it's a widely used starting estimate, not a precise prediction.
- How accurate is the estimated due date?
- Only about 1 in 20 babies actually arrive on their exact estimated due date. Most births happen within a couple of weeks either side. A first-trimester ultrasound (usually around weeks 8-12) often gives a more accurate estimate than LMP alone, especially for irregular cycles.
- What happens at each milestone scan?
- The dating/heartbeat scan (~week 8) confirms the pregnancy and estimated due date. The nuchal translucency scan (~week 12) screens for chromosomal conditions. The anomaly scan (~week 20) checks the baby's development in detail. Exact timing and availability of these scans varies by healthcare provider and country.
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.
- How is the due date calculated?
- The due date is the first day of your last menstrual period plus 280 days (40 weeks), following Naegele's rule. This assumes a typical 28-day menstrual cycle with ovulation around day 14; irregular cycles can shift the actual due date earlier or later than this estimate.
- What is Naegele's rule?
- Naegele's rule, named after 19th-century German obstetrician Franz Naegele, is the standard formula for estimating a due date: add one year, subtract three months, and add seven days to the first day of the last menstrual period — mathematically equivalent to adding 280 days.
- What counts as a full-term pregnancy?
- A full-term pregnancy is generally defined as delivery between 39 weeks 0 days and 40 weeks 6 days. Birth from 37 weeks 0 days to 38 weeks 6 days is considered "early term," and the broader normal range for birth spans roughly 37 to 42 weeks.
- What is the difference between gestational age and fetal age?
- Gestational age is counted from the first day of the last menstrual period, which is about two weeks before actual conception — this is the standard measure used by healthcare providers and by this calculator. Fetal age (or conceptional age) is counted from the actual date of conception, and runs roughly two weeks behind gestational age.