Conception Date Calculator
Enter your due date or last menstrual period (LMP) to estimate when conception likely occurred. Based on standard 28-day cycle and 280-day gestation. Results are estimates; consult your healthcare provider for precise dating.
Conception date estimation works backward from either your due date or your last menstrual period (LMP) using standard obstetric conventions. Conception itself — the actual moment a sperm fertilizes an egg — typically occurs about 14 days after the first day of the last menstrual period in a "textbook" 28-day cycle, coinciding with ovulation. The standard 280-day pregnancy (40 weeks) is measured from LMP, not from conception, which makes pregnancy "266 days from conception" or "280 days from LMP."
Why two different counting systems? Historically, obstetricians used LMP as the reference point because women typically know their period dates more reliably than their conception dates (and conception isn't directly observable). The 280-day "gestational age" framework became standard. But for women trying to establish paternity, calculate exact conception, or align their pregnancy with specific events, working back to the estimated conception date is meaningful.
This calculator provides conception date estimates from either due date (subtracts 266 days = 38 weeks) or LMP (adds 14 days = typical ovulation). Important caveats: these are estimates assuming a 28-day cycle with day-14 ovulation, both of which vary substantially in real cycles. Actual ovulation timing varies ±3-5 days in normal cycles, and sperm can live in the reproductive tract up to 5 days before fertilizing an egg. Combined, actual conception can occur anywhere within a roughly 7-10 day window. For more precise dating, first-trimester ultrasound (especially crown-rump length measurement at 8-12 weeks) is the obstetric gold standard, accurate to within 3-5 days. Consult your obstetrician for clinical decisions; this calculator is for general curiosity and rough planning.
Inputs
Results
Estimated Conception
Monday, September 22, 2025
Due Date
Monday, June 15, 2026
LMP
Monday, September 8, 2025
Pregnancy Timeline
| Milestone | Date |
|---|---|
| Last Menstrual Period | Monday, September 8, 2025 |
| Estimated Conception | Monday, September 22, 2025 |
| Estimated Due Date | Monday, June 15, 2026 |
| Conception Window | Friday, September 19, 2025 to Thursday, September 25, 2025 |
| 1st Trimester Ends | Monday, December 1, 2025 |
| 2nd Trimester Ends | Monday, March 16, 2026 |
Formula
How to use this calculator
- Choose your starting point: if you know your due date (often from early ultrasound), select that. If you know your last menstrual period (LMP), select that.
- Enter the relevant date.
- Review the estimated conception date.
- Account for cycle length: if your cycles are longer than 28 days, actual conception likely occurred a few days later than the estimate. Shorter cycles → earlier conception.
- For more precise dating: first-trimester ultrasound is the gold standard, accurate to ±3-5 days. Ask your obstetrician about your CRL-based dating.
- Remember the conception window: actual conception can occur ±5 days from the estimate due to ovulation timing variability and sperm viability (sperm can live 5 days in the reproductive tract before fertilizing an egg).
- For paternity questions: legal paternity determination requires DNA testing, not date calculations. Conception date estimates are not legally definitive.
- For early-pregnancy planning: pair with due-date calculator and pregnancy-weight-gain calculator for fuller pregnancy planning.
Worked examples
Standard pregnancy dating from due date
Due date: June 15, 2026. Conception estimate: June 15 − 266 days = September 22, 2025. LMP estimate: June 15 − 280 days = September 8, 2025. The 14-day difference between LMP and conception reflects the typical interval between menstruation onset and ovulation in a 28-day cycle. Both are useful reference points for different purposes.
LMP with longer cycle
LMP: September 8, 2025. Cycle length: 32 days (longer than standard 28). 28-day assumption: Conception estimate September 22, 2025; Due date June 15, 2026. 32-day adjusted: Ovulation likely around day 18 (LMP + 18), so conception ≈ September 26, 2025. Due date adjusted ~June 19, 2026. Longer cycles delay ovulation and shift both conception and due date. Many fertility apps and pregnancy calculators don't account for this, leading to "wrong" dating that ultrasound later corrects. If your cycles are typically 30-35 days, expect the formula-based due date to be ~3-5 days too early.
Conception window uncertainty
Estimated conception date: September 22, 2025. Actual window: roughly September 17-27, 2025. This range reflects: - Ovulation timing variability (±3 days) - Sperm viability (sperm from intercourse 1-5 days BEFORE ovulation can still fertilize) - Cycle-to-cycle variation even in "regular" cycles For paternity questions: this 10-day window means specific date attribution is difficult without DNA testing. Conception date calculators are not legally definitive for paternity. Use DNA testing for paternity determination. For event-correlation curiosity: the rough date is usually close enough for "we conceived during our vacation in late September" purposes.
When to use this calculator
Use this calculator out of curiosity about when conception likely occurred, when planning pregnancy timing, when correlating pregnancy with specific life events, or for general pregnancy education.
Pair with due-date calculator and ovulation-calculator for fuller fertility and pregnancy planning.
Important practical considerations:
1. **Estimates have inherent uncertainty.** Standard 28-day cycle assumptions don't apply to many women. Real conception windows span 7-10 days even in "regular" cycles. Date precision beyond ±5 days requires ultrasound.
2. **Ultrasound is more precise.** First-trimester ultrasound (8-12 weeks) crown-rump length measurement is accurate to ±3-5 days regardless of cycle regularity. This is the obstetric standard for pregnancy dating and supersedes LMP-based estimates when discrepant.
3. **Don't use for paternity determination.** Conception date estimates are NOT legally definitive. The 7-10 day conception window often spans periods when multiple partners may have been involved. DNA paternity testing (after birth, or via non-invasive prenatal paternity testing using maternal blood ~9+ weeks) is the only definitive method.
4. **Cycle length matters.** If your typical cycles are not 28 days, adjust expectations: longer cycles → later conception/due date than formula suggests; shorter cycles → earlier. Some apps account for this; many don't.
5. **Ovulation can vary cycle-to-cycle.** Even women with "regular" cycles ovulate ±3-5 days from their typical day. Stress, illness, travel, weight changes, and hormonal shifts all affect timing.
6. **Two-week wait.** Conception happens during the fertile window (5 days before through day of ovulation), but pregnancy isn't detectable for about 2 weeks after. Implantation occurs ~6-12 days after conception; hCG becomes detectable ~10-14 days after conception. Early home pregnancy tests can detect from ~10-14 DPO (days past ovulation).
7. **Gestational age vs. fetal age.** Gestational age (counted from LMP) is the universal obstetric standard — when a doctor says "you're 8 weeks pregnant," they mean 8 weeks gestational age (~6 weeks since conception). Fetal age (from conception) is ~2 weeks less. Don't confuse them.
8. **Date adjustment after ultrasound.** If first-trimester ultrasound shows the baby measuring meaningfully larger or smaller than LMP-based dating predicts (>5-7 days), obstetricians typically revise the due date to match ultrasound. This is more accurate than calendar-based estimation.
Common mistakes to avoid
- Treating the conception date as precise. Estimates have ±5 day inherent uncertainty due to ovulation timing variability and sperm viability.
- Using for paternity determination. Conception date calculators are not legally definitive. Use DNA testing for paternity.
- Ignoring cycle length. Longer or shorter cycles than 28 days shift the conception window. Formula-based estimates assume 28-day cycles.
- Confusing gestational age with fetal age. Gestational age (from LMP) is the obstetric standard; fetal age (from conception) is ~2 weeks less. "8 weeks pregnant" means 8 weeks gestational age.
- Trusting calendar dating over ultrasound. First-trimester ultrasound is more precise than LMP-based dating. When they disagree, ultrasound wins for clinical purposes.
- Assuming day-14 ovulation. Many women ovulate days earlier or later than day 14. Cycle tracking, ovulation predictor kits, or BBT charting reveal individual timing.
Frequently Asked Questions
Sources & further reading
- Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists
- Pregnancy — Reproductive Health — U.S. Centers for Disease Control and Prevention
- Reproductive Health Information — U.S. National Institute of Child Health and Human Development