# How Due Date Calculators Work (And Why They're Just Estimates) _The math, the methods, and what to do when your provider gives you a different date._ - Source: https://proba.baby/en/blog/how-due-date-calculators-work - Published: 2026-09-05 - Author: ProbaBaby Team - Category: Calculators & Tools - Tags: calculators, first-trimester - Language: English > Most people who Google "due date calculator" want their EDD in 10 seconds. Here's what's actually happening when you punch in a date — and why your provider's first-trimester ultrasound is the only date that really counts. > **Medical disclaimer.** This article is for general information and does not replace medical advice. Always discuss your specific pregnancy timeline with your healthcare provider. If you've just seen a positive pregnancy test, "when is my baby due?" is probably the first thing your brain offers up. A due date calculator can answer that in seconds — but the answer is an estimate, not a deadline. Only about **4% of babies arrive exactly on their estimated due date** (Jukic 2013). Most arrive in a two-week window before or after. So what is the calculator actually doing? It's running one of three formulas, picked based on which date you know. Here they are, in plain language. ## Method 1 — Naegele's Rule (LMP-based) This is the formula your provider will use at your first appointment if you have a regular cycle. It dates back to 1812, named after Franz Karl Naegele, a German obstetrician. The rule: **EDD = first day of last menstrual period + 280 days.** That's 40 weeks. The clock starts two weeks before you actually conceived — Naegele's rule assumes a 28-day cycle with ovulation on day 14, so the first two weeks of "pregnancy" happen before fertilization. Weird, but it's the standard. This is what our [Due Date Calculator](/tools/due-date-calculator) uses when you pick **Last period** mode. ### What if your cycle isn't 28 days? Naegele's rule loses accuracy. If your cycles consistently run 32 days, your true conception date is later than day 14, so your due date is later too — by about four days. If your cycles run 25 days, you ovulate earlier, so your due date is earlier. A first-trimester ultrasound corrects for this. ## Method 2 — Conception dating If you know the day you conceived (planned cycle, ovulation tracking, fertility awareness), you can skip Naegele's rule entirely: **EDD = conception date + 266 days.** That's the actual gestational period — 38 weeks from fertilization to birth. The 14-day offset built into LMP dating disappears here. This is the more biologically honest formula, but most people don't know their conception date precisely. They know when their period stopped. That's why LMP dating is the medical default. ## Method 3 — IVF dating For IVF pregnancies, the calendar is exact: you know the day the embryo was transferred and how old the embryo was at transfer. The ASRM (American Society for Reproductive Medicine) formula: - **Day-3 transfer:** EDD = transfer date + 263 days - **Day-5 transfer (blastocyst):** EDD = transfer date + 261 days The difference is that an embryo's developmental clock starts at fertilization, not at the transfer. A day-3 embryo has been developing for 3 days when it goes in; a day-5 blastocyst for 5. Subtract those days from 266 (the conception-to-birth period) and you get the transfer-to-birth period. ## So which method is most accurate? **None of them, really — early ultrasound is more accurate than any of these formulas.** A first-trimester ultrasound (usually between 8 and 13 weeks) measures the crown-rump length of the embryo and uses that to date the pregnancy. If the ultrasound estimate differs from the LMP-based estimate by **more than 7 days**, ACOG recommends using the ultrasound date going forward. Your provider may give you a slightly different due date after that scan. That's normal — they're using the most precise data available. After the first trimester, ultrasound accuracy drops. Second-trimester ultrasounds can be off by 14–21 days; third-trimester by 21 days or more. So if your first scan happens late, the LMP estimate may stick. ## Why does it matter if it's an estimate? A few reasons: - **Scheduling tests** — first-trimester screening, NIPT, anatomy scan, glucose tolerance test, and Group B strep test are all timed to gestational age, not to the calendar. An off-by-a-week dating error can push tests outside their valid window. - **Preterm vs full-term cutoffs** — labor before 37 weeks is preterm. If your dating is off, a "preterm" baby might actually be 37 weeks old, or vice versa. - **Induction timing** — most providers offer induction around 41 weeks. If your dating is wrong by a week, you might be induced "too early" or "too late." This is why your provider may revise your due date even when your math feels right. They're not second-guessing you — they're using the most precise tool they have. ## What about "I'm 5 weeks pregnant" — when did that start? By LMP dating, you're 5 weeks pregnant **5 weeks after the first day of your last period**. The actual embryo is about 3 weeks old at that point — you conceived 2 weeks into the count, then it took a few days for the embryo to implant. Most home pregnancy tests turn positive around the time you'd expect your next period, which is gestational week 4. That's why "I just found out I'm pregnant" usually means "I'm about 4–5 weeks along." You're not as early as it sounds. ## Late vs. post-term: how late is too late? The clinical naming: - **Preterm** — before 37 weeks - **Early term** — 37 weeks 0 days to 38 weeks 6 days - **Full term** — 39 weeks 0 days to 40 weeks 6 days - **Late term** — 41 weeks 0 days to 41 weeks 6 days - **Post-term** — 42 weeks 0 days onward Most providers will discuss induction at 41 weeks; most will recommend it at 42. The risk profile changes after 42 weeks, but most births happen between 38 and 41 weeks regardless. ## How ProbaBaby helps ProbaBaby's pregnancy module starts with your EDD (calculated the same way as the tool above), then automatically re-anchors it if you log an updated date from your provider. Every weekly checklist, symptom prediction, and appointment reminder updates with the new date — no manual recalculation. If you have a first-trimester ultrasound that shifts your date, it just works. The week-by-week timeline, the trimester transitions, the kick-counter prompts at week 28, the hospital-bag nudge at week 32, the contraction timer ready at week 36 — they all key off whatever your current EDD is, even if it's been revised twice. [**Try the free due date calculator now →**](/tools/due-date-calculator) ## Quick FAQ **Why is my LMP-based EDD different from my ultrasound EDD?** Because Naegele's rule assumes a 28-day cycle and your cycle may not match. Trust the ultrasound if the difference is >7 days in the first trimester. **Can my due date change later in pregnancy?** Yes — though it rarely changes after the first trimester. Late-pregnancy ultrasounds have ±2–3 weeks of error, so providers stick with the earlier dating. **My calculator says I'm 6 weeks but my doctor says 5. Who's right?** Probably your doctor. They may have used your ultrasound to date you. The calculator only uses the inputs you give it. **Does conception date give me a more accurate due date than LMP?** Yes, if you actually know the conception date (tracked ovulation, planned cycle). For most people, LMP is the more reliable input because it's easier to remember. **Is it possible to be more than 42 weeks pregnant?** Technically yes, but at that point your provider has almost certainly recommended induction. True 42+ week pregnancies are uncommon. ## Sources - ACOG Committee Opinion No. 700: Methods for Estimating the Due Date. Available at [acog.org](https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date) - American Society for Reproductive Medicine (ASRM) Practice Committee documents on IVF dating - NIH MedlinePlus: Pregnancy basics — [medlineplus.gov/pregnancy.html](https://medlineplus.gov/pregnancy.html) - Mayo Clinic: Prenatal care — first trimester pregnancy --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.