Conception Calculator — When Did I Conceive? (Estimate Conception Date)
Estimate your conception date based on due date, last period, or ultrasound date. Understand the fertile window and how conception timing is calculated.
"When did I actually conceive?" is one of the most common pregnancy questions — and the answer is almost never what people expect. Conception doesn't happen on the day you had intercourse; sperm can survive up to 5 days, and the egg lives 12–24 hours. The actual fertilization usually happens 1–3 days after intercourse.
The CalcHub Conception Calculator estimates your likely conception date from your due date, last menstrual period, or ultrasound dating.
Three Ways to Estimate Conception Date
Method 1: From Your Due Date
Conception date ≈ Due date − 266 days (38 weeks)| Due Date | Estimated Conception |
|---|---|
| January 15 | April 24 (previous year) |
| March 20 | June 27 (previous year) |
| July 10 | October 17 (previous year) |
| October 5 | January 12 (same year) |
| December 25 | April 3 (same year) |
Method 2: From Your Last Menstrual Period (LMP)
Conception date ≈ LMP + 14 days (for a 28-day cycle)This assumes ovulation occurred on day 14 of your cycle. For longer cycles, add more days:
| Cycle Length | Likely Ovulation Day | Conception Window |
|---|---|---|
| 26 days | Day 12 | Days 7–13 after LMP |
| 28 days | Day 14 | Days 9–15 after LMP |
| 30 days | Day 16 | Days 11–17 after LMP |
| 32 days | Day 18 | Days 13–19 after LMP |
| 35 days | Day 21 | Days 16–22 after LMP |
Method 3: From Ultrasound Dating
Early ultrasounds (6–10 weeks) are the most accurate dating method. Your doctor measures the embryo's crown-rump length and estimates gestational age ± 3–5 days. Subtract 2 weeks from gestational age to estimate conception. Example: Ultrasound says 8 weeks 3 days on March 15 → conception was approximately January 19 (8 weeks 3 days − 14 days ≈ 6 weeks 3 days before March 15).Why Conception Date Isn't the Same as Intercourse Date
| Event | When It Happens |
|---|---|
| Intercourse | Day 0 |
| Sperm reaches fallopian tube | 30 min – 12 hours later |
| Sperm waits for egg | Up to 5 days |
| Ovulation (egg released) | Within the fertile window |
| Fertilization (conception) | Within 12–24 hours of ovulation |
| Implantation | 6–10 days after fertilization |
Conception Date by Due Date — Quick Reference
| Due Date Month | Conception Month |
|---|---|
| January | April (previous year) |
| February | May (previous year) |
| March | June (previous year) |
| April | July (previous year) |
| May | August (previous year) |
| June | September (previous year) |
| July | October (previous year) |
| August | November (previous year) |
| September | December (previous year) |
| October | January |
| November | February |
| December | March |
How to Use the Calculator
- Open the CalcHub Conception Calculator
- Choose your method: due date, last period, or ultrasound date
- Enter your cycle length (if using LMP method)
- See: estimated conception date and likely conception window (range of days)
Accuracy
| Method | Accuracy |
|---|---|
| Early ultrasound (6–10 weeks) | ± 3–5 days |
| LMP (regular 28-day cycles) | ± 5–7 days |
| LMP (irregular cycles) | ± 7–14 days |
| Due date calculation | ± 5–7 days |
Can I use the conception calculator to determine paternity?
The calculator gives a window of 5–7 days, not an exact date. If intercourse with different partners occurred within this window, conception dating alone cannot determine paternity. A DNA test is the only definitive method.
Does IVF change the conception date?
For IVF, the "conception date" is known precisely — it's the date of embryo transfer (for fresh transfers) or egg retrieval (for frozen transfers). Due date = transfer date + 266 days (for Day 0 transfer) or adjusted for embryo age.
My doctor's due date doesn't match my calculation. Which is right?
Trust the ultrasound dating, especially from early scans (6–10 weeks). LMP calculations assume a 28-day cycle with ovulation on day 14, which doesn't apply to everyone. Ultrasound measurements are more accurate for individual pregnancies.
Related Calculators
- Pregnancy Due Date Calculator — estimate delivery date
- Ovulation Calculator — find your fertile window
- Period Calculator — track your menstrual cycle
- Age Calculator — exact age calculation