Conception Date Calculator
Working backward from a known or estimated due date, conception typically occurred 266 days (38 weeks) earlier. This calculator estimates the conception date range based on days until the due date, useful for family planning and retrospective pregnancy dating.
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Formula
Conception date = Due date − 266 days (38 weeks from fertilization)
Human gestation from fertilization to birth averages 266 days (38 weeks). Due dates calculated by Naegele's Rule add 280 days to LMP, which includes approximately 14 days before ovulation and conception. Subtracting 266 from the due date in days-from-now gives the day number relative to today when conception likely occurred — negative values mean conception was in the past.
How to use the Conception Date Calculator
- 1
Enter your days until due date
Value should be in days.
- 2
Read your results instantly
Results update in real time as you type.
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Conception versus gestational age dating
Obstetricians date pregnancies from the last menstrual period (LMP), not from conception, because LMP is a known date while conception is rarely observed. This creates the 2-week offset: gestational age (from LMP) is always about 2 weeks more than fetal age (from conception). A pregnancy described as 10 weeks gestational age had conception approximately 8 weeks ago. This calculator uses 266 days from conception — equivalent to 280 days from LMP — to align with standard obstetric due date calculation.
Why conception dating has variability
Sperm can survive in the reproductive tract for up to 5 days, meaning conception can occur 1–5 days after intercourse. Ovulation itself may vary by several days from cycle to cycle. This creates a conception window of up to 6 days even for a single cycle, making the exact conception date inherently uncertain. First-trimester ultrasound, which measures crown-rump length, provides a more accurate gestational age than either LMP or retrospective conception dating.
Tips & Insights
Conception date is approximate by nature
The 266-day figure is an average. Individual pregnancies range from 259 to 294 days at term. Use this as a historical estimate rather than a precise date.
Cross-reference with LMP if known
If you know your LMP, use the due date calculator on this site to confirm your due date, then use the conception calculator to work backward. Consistency between the two approaches validates the estimate.
Ultrasound dating supersedes formula dating
If a first-trimester ultrasound has been performed, use that gestational age as the reference point — it is more accurate than any formula-based estimate including this one.
Worked Examples
Due in 200 days
Conception was approximately 66 days ago
Due in 280 days (just found out)
Conception was approximately 14 days ago (about 2 weeks ago)
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Frequently Asked Questions
Why 266 days and not 280?
280 days is measured from LMP; conception occurs approximately 14 days after LMP (at ovulation). So fetal age from conception is 280 − 14 = 266 days — the actual time from fertilization to birth.
Can I use this to determine paternity?
No. Conception date estimates have a margin of error of ±1–2 weeks due to cycle variability and sperm survival. Paternity determination requires DNA testing, not date-based calculations.
What if my due date was changed by ultrasound?
Use the ultrasound-adjusted due date as your input. First-trimester ultrasound is more accurate than LMP dating, and the conception date estimate will shift accordingly.
Is the conception date the same as the fertilization date?
Essentially yes. Fertilization occurs within hours to days of ovulation. For practical purposes, conception and fertilization date are treated as the same event.
Can I get pregnant outside my estimated conception window?
The conception window is 5–6 days per cycle (sperm survival + egg viability). Pregnancies do not occur outside of this window for a given cycle, though predicting the exact window is imprecise without LH testing or ultrasound.
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