The due date obsession starts with Naegele's rule, published in 1830 by German obstetrician Franz Naegele: add 280 days (40 weeks) to the first day of the last menstrual period. It assumes a 28-day cycle with ovulation on day 14 — which describes only about 30% of women. Yet it remains the default because it's simple, cheap and close enough for most patients.

The reality is broader. Only about 4% of babies arrive on the exact predicted due date. Most (roughly 75%) arrive within a two-week window centered on the EDD. Full term is 37–41 weeks. Post-term (over 42 weeks) is where obstetric intervention becomes standard — stillbirth risk rises measurably after 41+3 to 42 weeks, so most providers offer induction in that window.

For women with irregular cycles or unknown LMP, ultrasound-based dating is more accurate. A first-trimester ultrasound (6–13 weeks) measures crown-rump length — a highly consistent embryonic dimension — and dates the pregnancy within ±3–5 days. After 20 weeks, ultrasound dating loses accuracy (±2 weeks) because normal variation in fetal size increases. Modern practice: adjust EDD to match ultrasound if LMP-based date differs by more than 5–7 days early on.

IVF pregnancies get the most precise dating. The conception date is known exactly (day of egg retrieval or embryo transfer). Day 3 embryo transfer: due date = transfer date + 263 days. Day 5 blastocyst transfer: transfer date + 261 days. This precision matters for scheduling C-sections, monitoring high-risk pregnancies and interpreting ultrasound growth curves.

Beyond the number, the calculator's real value is the timeline. Weekly milestones tell you what's developing (heartbeat detectable by transvaginal ultrasound at 6 weeks, all major organs formed by 12 weeks, viability outside the womb around 24 weeks). Prenatal test scheduling aligns with gestational age: NIPT at 10+ weeks, anatomy scan at 18–22 weeks, glucose tolerance test at 24–28 weeks, GBS swab at 35–37 weeks. Knowing your exact week converts pregnancy from anxiety into a series of expected checkpoints — one calculator, forty weeks of planning.