Key takeaways
- Five evidence-based dating methods: last menstrual period (LMP), conception date, first-trimester ultrasound (CRL or weeks+days), IVF transfer date, or reverse from a known due date.
- IVF dating is the most precise (transfer date is known to the day). First-trimester CRL ultrasound is the next most reliable (±5 days at 7–12 weeks).
- LMP-based Naegele's rule is the standard fallback when ultrasound is unavailable. Adjusts automatically for non-28-day cycles.
- Output includes EDD, gestational age today, three trimester boundaries, and key clinical milestones (heartbeat, dating scan, anatomy scan, viability, full-term).
- Calculations run on your device. Share a result by copying the URL — nothing is stored on a server.
How it works
Pregnancy dating starts with a single goal: pinning down when conception happened. Once you know that, the rest is simple arithmetic — full-term pregnancy is approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP), or about 38 weeks (266 days) from conception.
The challenge is that most users don't know the conception date precisely. So we offer five methods, ranked by reliability:
- IVF transfer date (most precise). Conception is known to the day. EDD = transfer + 266 − embryo age in days. We support Day-3 (cleavage), Day-5 (blastocyst), and Day-6 (delayed blastocyst) transfers.
- First-trimester ultrasound (CRL). An early ultrasound measures crown-rump length, which we convert to gestational age via the Robinson–Fleming formula. Most accurate at 7–12 weeks (±5 days).
- First- or second-trimester ultrasound (weeks+days). If your sonographer gave you a gestational age in weeks and days, we use that directly.
- Conception date. Reliable only when ovulation was confirmed (LH test, BBT chart, or known intercourse window). EDD = conception + 266 days.
- LMP (Naegele's rule). EDD = LMP + 280 days. We adjust for non-28-day cycles by adding (cycle − 28) days.
- Reverse from a known EDD. If you already have an EDD from your provider and want to back out your LMP and conception date, use this method.
We display your EDD, your gestational age today (in weeks and days), and three milestone groups: trimester boundaries, key clinical milestones (dating scan, anatomy scan, viability, full-term), and a fertility/pregnancy-week breakdown.
The science behind it
Naegele's rule and where it came from
Franz Karl Naegele was a 19th-century German obstetrician who codified the rule that bears his name: EDD = LMP + 9 months + 7 days, or equivalently, LMP + 280 days. The math assumes a regular 28-day cycle and ovulation on day 14. It is remarkably durable as a rough estimate — within ±7 days for regular cycles — and is the foundation of LMP-based pregnancy dating worldwide.
The key limitation: ovulation does not always occur on day 14. In cycles longer or shorter than 28 days, ovulation is correspondingly later or earlier. We adjust by adding (cycle − 28) days to the standard 280, which is the standard correction.
Robinson–Fleming CRL formula
First published in 1975 in the British Journal of Obstetrics and Gynaecology, the Robinson–Fleming formula remains the most widely used CRL-to-gestational-age conversion:
Gestational age (days) = 8.052 × √CRL(mm) + 23.73
It is most accurate when CRL is between 10 mm and 84 mm (corresponding to gestational ages of roughly 7 to 14 weeks). Outside that range, the formula loses precision; clinicians switch to head-circumference or biparietal-diameter measurements after 14 weeks.
Why ultrasound dating wins after a disagreement
ACOG Committee Opinion 700 is the authoritative US guidance on pregnancy dating. The summary: if first-trimester ultrasound and LMP disagree by more than 5–7 days, the ultrasound is more reliable and should set the EDD. Late ultrasounds (after 22 weeks) are too imprecise to override an earlier dating estimate.
For practical purposes: trust your provider's first-trimester ultrasound over any calculator that disagrees, including this one. We surface the LMP-derived EDD as a useful secondary anchor, not as a competing claim.
IVF dating is the gold standard — and that's not hyperbole
For IVF pregnancies, conception timing is known with unusual precision. Egg retrieval, the identifiable point of fertilization, and embryo transfer are documented to the day. The "LMP" used for IVF dating is a back-calculated date (transfer minus the embryo age plus 14 days), not the patient's actual last period. EDD is then computed normally: LMP + 280 days, or equivalently, transfer + 266 − embryo age. For a Day-5 blastocyst transfer, EDD = transfer + 261.
The "post-term" cliff at 42 weeks
While the EDD is at 40 weeks, "term" extends to 42 weeks. Most providers begin discussing induction at 41 weeks because risk of stillbirth and meconium aspiration begins to rise after 42. Only about 5% of pregnancies last past 42 weeks; many of these are dating errors rather than truly delayed deliveries.
References
- ACOG Committee Opinion 700: Methods for Estimating the Due Date — ACOG, 2017 (reaffirmed 2024)
- A critical evaluation of sonar "crown-rump length" measurements — Robinson HP, Fleming JE., BJOG 1975
- Estimating gestational age from ultrasonographic measurements — Hadlock FP et al., Radiology 1991
- IVF and gestational dating — ASRM Practice Committee
How to use this calculator
- Pick the dating method that matches your data. IVF transfer date if you conceived through IVF. CRL or weeks+days if you have a first-trimester ultrasound. Conception date only if ovulation was confirmed (BBT, LH test, or known intercourse window). Otherwise LMP.
- Enter the relevant date. For LMP, the first day of your last period. For ultrasound, the date of the scan. For IVF, the day of transfer. For reverse, your provider's EDD.
- Adjust the cycle length if you know it. Naegele's rule assumes a 28-day cycle. Longer cycles push the EDD later by the same number of days; shorter cycles pull it earlier.
- For ultrasound, enter CRL or weeks+days. Use crown-rump length (CRL) in millimeters if your scan was at 7–14 weeks. Use weeks+days if your scan was beyond that.
- Read your result. You will see your EDD, gestational age today, your three trimesters, and the major prenatal milestones ahead of you.
Limitations & medical disclaimer
- EDD is a midpoint estimate. Only ~4% of births happen exactly on the EDD; about 90% happen between 38 and 42 weeks.
- LMP-based dating assumes a regular 28-day cycle and ovulation on day 14. For irregular or long cycles, request a first-trimester dating ultrasound.
- Ultrasound dating is most accurate at 7–12 weeks (±5 days). After 22 weeks, ultrasound dating carries ±10–14 day uncertainty — defer to the earliest reliable dating scan.
- For IVF pregnancies, always prefer IVF dating over LMP. Conception timing is known precisely.
- This calculator does not replace your prenatal care provider. If your provider sets an EDD that differs from this calculation, follow their EDD.
HerCalc tools are educational and do not replace professional medical advice. Always consult a qualified clinician for diagnosis or treatment decisions.
Frequently asked questions
Which due-date method is most accurate? +
In order of accuracy: (1) IVF transfer date — exact, since the conception date is known to the day. (2) First-trimester ultrasound at 7–12 weeks — accurate to within 5 days. (3) Conception date with confirmed ovulation. (4) LMP-based Naegele's rule — accurate within ±7 days for regular cycles. After 22 weeks, ultrasound dating becomes much less reliable (±10–14 days). Always defer to your provider's decision; if there's a disagreement, the earliest reliable dating scan usually wins.
Why does my calculated EDD differ from my doctor's? +
A few common reasons. Your doctor may be using your first-trimester ultrasound (more accurate than LMP) while this calculator is using your LMP. Your cycle length may differ from 28 days, and Naegele's rule has not been adjusted for it. There may be a few days' rounding difference between formulas. ACOG guidance: when LMP and first-trimester ultrasound disagree by more than 5–7 days, the ultrasound is the better estimate.
How accurate is Naegele's rule for cycles that aren't 28 days? +
Naegele's rule (LMP + 280 days) assumes a 28-day cycle with ovulation on day 14. For cycles longer than 28 days, ovulation is later, and EDD shifts later by roughly the same number of days. This calculator adjusts automatically when you enter a non-28-day cycle. For cycles that vary significantly month-to-month or for PCOS, Naegele's rule is unreliable — request a first-trimester dating ultrasound.
I conceived through IVF. How is the EDD calculated? +
IVF dating is the most precise method available because the conception event is known to the day. The formula: EDD = transfer date + 266 − embryo age in days. For a Day-5 blastocyst transfer, that's transfer + 261 days. For Day-3, transfer + 263. For Day-6 (rare), transfer + 260. We implement this internally as LMP = transfer − (14 + embryo age) and EDD = LMP + 280, which is mathematically equivalent and lets the rest of the tool (gestational age, trimesters) work as expected.
What is CRL and why does my ultrasound show it? +
Crown-Rump Length (CRL) is the distance from the top of the embryo's head to its rump, measured on early ultrasounds (typically 6–14 weeks). CRL is the most accurate predictor of gestational age in the first trimester because early embryos grow at a remarkably consistent rate. The formula we use is from Robinson and Fleming (1975): gestational age in days = 8.052 × √CRL(mm) + 23.73. Most clinics use a similar or identical formula.
Will my baby actually be born on the due date? +
Only about 4% of births happen on the exact due date. About 50% of first-time pregnancies deliver within 5 days of EDD; about 90% deliver between 38 and 42 weeks. The "due date" is a midpoint estimate, not a deadline. Most providers use the EDD to schedule prenatal visits and to define the boundaries of "term" (37+0 to 41+6). Induction for post-term pregnancy is typically discussed around 41 weeks.
Can the EDD change later in pregnancy? +
Yes — and this is normal. If your first-trimester dating scan disagrees with your LMP-based EDD by more than 5–7 days, your provider will likely reset the EDD to match the ultrasound. Late ultrasounds (after 22 weeks) carry too much measurement uncertainty to override an earlier estimate. Once the EDD is confirmed by an early scan, it does not change for the rest of the pregnancy regardless of fetal size.
What happens at each trimester? +
Trimester 1 (weeks 0–13): organogenesis — the most sensitive period. First prenatal visit, dating scan around 8–10 weeks, NIPT genetic screening from 10 weeks. Common symptoms: nausea, fatigue, breast tenderness.
Trimester 2 (14–27): the "honeymoon" trimester. Anatomy scan at 18–22 weeks, often with sex reveal. First fetal movements 18–22 weeks. Glucose tolerance test for gestational diabetes at 24–28 weeks.
Trimester 3 (28–40): preparation. Group B strep swab at 35–37 weeks. Daily fetal-movement counting. Hospital bag by week 36. Most births happen between 38 and 42 weeks.