Conception Date Calculator
Estimates the most likely conception date by counting back 266 days (38 weeks) from your due date. Useful when you know your due date but want to narrow down when fertilization likely occurred.
Last updated: May 2026
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About this calculator
Conception occurs approximately 14 days after the first day of the last menstrual period in a standard 28-day cycle — or about 266 days (38 weeks) before the due date. Because the EDD is conventionally set at 280 days from LMP, subtracting 14 days gives the estimated conception window: conception_date ≈ EDD − 266 days. In this calculator the input is expressed as days remaining until the due date, so days from today to conception = days_until_due − 266 (a negative value means conception already occurred that many days ago). Variables: dueDate (days remaining until your estimated due date). Edge cases: the 266-day figure assumes a regular 28-day cycle with ovulation on day 14. For shorter cycles (21–24 days), ovulation may occur as early as day 7, shifting conception 4–7 days earlier than the formula suggests. For longer cycles (32+ days), conception may occur up to 7 days later. Sperm survive 3–5 days in the female reproductive tract while the egg is viable for only 12–24 hours, so the actual fertilization window is a 5–6 day range, not a single day. IVF pregnancies have a known conception (transfer) date that overrides this estimate. The formula is conventionally accurate to within 5–7 days for cycle-regular women; ultrasound dating is more precise for clinical purposes. The conception estimate cannot determine paternity — only DNA testing can.
How to use
Example 1: Your due date is 200 days from today. Step 1: days from today to conception = 200 − 266 = −66, meaning conception occurred approximately 66 days ago. Step 2: convert to weeks pregnant — gestational age = 14 weeks (since LMP was ~80 days ago, conception ~66 days ago, fetal age ~9.4 weeks). Verify: if today is May 15, count back 66 days → conception around March 10; LMP would have been ~14 days earlier, around February 24. Example 2: Your due date is 130 days from today. Step 1: 130 − 266 = −136 days, so conception was approximately 136 days ago. Step 2: gestational age = (280 − 130) / 7 = 150 / 7 ≈ 21.4 weeks — you are early in the third quintile of pregnancy. Verify: 136 days = roughly 19.4 weeks since conception, which matches a gestational age of 21.4 weeks (fetal age is always ~2 weeks less than gestational age).
Frequently asked questions
How does a conception date calculator estimate when fertilization occurred?
The calculator subtracts 266 days from the due date because a full-term pregnancy from conception to delivery averages 38 weeks (266 days). The conventional 40-week gestational age is counted from the LMP, adding the ~14-day pre-ovulation window. Since ovulation and fertilization typically occur around cycle day 14, this approach places conception in the correct two-week window after the LMP. The result is a probabilistic estimate, not a precise date — sperm can survive 3–5 days in the reproductive tract, so the actual fertilization window spans several days centered on ovulation. Ultrasound crown-rump length measurement in the first trimester remains the most precise way to confirm gestational timing and indirectly the conception date.
Why is the conception date different from the last menstrual period date?
Clinicians date pregnancy from the LMP because it is a known, observable date that doesn't require predicting ovulation, whereas the exact moment of fertilization is almost never directly observable. In a textbook 28-day cycle, the LMP precedes conception by about 14 days. So while a 40-week pregnancy is counted from the LMP, the embryo itself is only about 38 weeks old at birth. This two-week offset is a built-in feature of all standard pregnancy dating systems worldwide and is why early ultrasounds may show 'no embryo visible' at 5 weeks gestational age — at that point the embryo is only 3 weeks old. Fetal age is sometimes used in scientific contexts but never in routine clinical practice.
Can the conception date calculator tell me who the father is?
No — the conception date calculator provides only a probabilistic window, typically a 5–7 day span centered on the most likely ovulation date. If intercourse with more than one partner occurred within that window, the calculator cannot determine paternity. DNA paternity testing (typically with 99.99%+ accuracy on a 16-marker panel) is the only scientifically reliable method to establish biological parenthood. The conception window can help you assess whether timing is plausible for a specific encounter, but timing alone is never definitive. Non-invasive prenatal paternity testing using fetal DNA in maternal blood is also available from around 7–9 weeks gestation if early paternity determination is needed. For legal or custody purposes, only laboratory DNA testing is admissible.
What are common mistakes when interpreting a conception date estimate?
Treating the estimated conception date as exact rather than as the center of a 5–7 day window leads to false confidence. Using the formula for women with irregular or long cycles (35+ days) makes the estimate several days too late. Forgetting that 'days until due date' must be measured precisely (counting today as day 0 vs day 1 shifts the result). Conflating fetal age with gestational age — the conception calculator gives fetal-age timing, while clinical conversations use gestational age. Ignoring the difference between ovulation date (when the egg is released) and fertilization date (when sperm meets egg) — they can differ by up to 24 hours. Finally, applying the formula to an IVF pregnancy ignores the known transfer date, which gives exact conception timing.
When should I NOT use this conception date calculator?
For IVF and other assisted-reproduction pregnancies, the embryo transfer date is the exact known conception date — no estimation needed. Women with cycles substantially different from 28 days (under 23 or over 35) should not use the standard formula without adjustment, as ovulation timing differs. After 13 weeks of pregnancy, ultrasound dating supersedes LMP-based estimates and should be used instead. For legal paternity questions, consult a court-admissible DNA testing service rather than relying on date estimates. For fertility-tracking purposes (trying to conceive), use an ovulation predictor or basal body temperature method that tracks actual cycle markers, not a back-calculation from a hypothetical due date. Pregnancies achieved while breastfeeding, perimenopausal, or just after hormonal contraception have such unpredictable ovulation that conception-date estimates are essentially guesses.