Conception Date Calculator
Not sure when conception happened? This calculator works backward from your due date or forward from your last menstrual period (LMP) to estimate the day fertilization likely occurred. It also shows a 5-day fertile window — the range during which intercourse could have led to pregnancy — and your current gestational age.
When to use this calculator
- Estimate conception date when the due date is known from an ultrasound
- Identify the fertile window around your likely ovulation date
- Confirm gestational age in weeks and days as of today
- Understand how a longer or shorter cycle shifts the conception estimate
- Cross-check conception timing for paternity or personal records
- Plan or document pregnancy milestones from a known LMP
How it works
2 min readWhat is conception date?
Conception date is the day a sperm fertilizes an egg, typically occurring around 14 days after the last menstrual period in a standard 28-day cycle. It falls approximately 266 days before the due date. Conception timing varies by cycle length, shifting the fertile window by 1–10 days, making it difficult to pinpoint without medical imaging.
How It Works
The calculation uses the Naegele's Rule framework combined with cycle-length adjustment.
Key biological facts
Formula
ovulation_day_from_lmp = cycle_length - 14
conception_date = LMP + ovulation_day_from_lmp
fertile_window_start = conception_date - 4 (days)
fertile_window_end = conception_date
EDD (due date) = LMP + 280 - (cycle_length - 28)When you enter a due date, the calculator first derives LMP:
LMP = EDD - 280 + (cycle_length - 28)Then applies the ovulation formula above.
Cycle-length adjustment
| Cycle length | Days from LMP to ovulation | Shift vs. 28-day |
|---|---|---|
| 21 days | 7 days | −7 days |
| 28 days | 14 days | 0 |
| 35 days | 21 days | +7 days |
Worked example
Suppose EDD = July 15, 2026 and cycle = 30 days.
1. LMP = July 15 − 280 + (30 − 28) = July 15 − 278 = October 10, 2025
2. Ovulation offset = 30 − 14 = 16 days
3. Conception date = Oct 10 + 16 = October 26, 2025
4. Fertile window = Oct 22 – Oct 26, 2025
5. If today is April 28, 2026: gestational age = (Apr 28 − Oct 10) = 198 days = 28 weeks 2 days
Gestational age
Gestational age is counted from LMP, not from conception, which is the standard used by OB-GYNs and ultrasound dating.
Limitations
Frequently asked questions
How accurate is the estimated conception date?
For women with regular 28-day cycles, the estimate is typically within 2–3 days. Cycle irregularity, late ovulation, or early implantation can shift the actual conception date by a week or more. First-trimester ultrasound dating (crown-rump length before 14 weeks) is more precise and should override this estimate if available.
Why is gestational age counted from LMP, not conception?
LMP is a date women can reliably recall, while the exact ovulation and fertilization date is rarely known. The medical standard established by ACOG counts pregnancy in weeks from LMP. This means gestational age at conception is roughly 2 weeks, even though the embryo is only days old.
What if my cycles are irregular?
The calculation becomes less reliable. If your cycles vary by more than 7 days month to month, enter your best average cycle length. Even then, consider the result a rough estimate. Ultrasound dating remains the gold standard for irregular-cycle pregnancies.
Can the fertile window extend beyond 5 days?
The 5-day window (4 days before ovulation through ovulation day) covers the highest-probability period based on sperm survival data. Some researchers include a 6th day (the day after ovulation) when the egg is still viable. This calculator uses the conservative 5-day range.
My due date changed after an ultrasound — which should I use?
Use the ultrasound-adjusted EDD, especially if the scan was done before 14 weeks. First-trimester ultrasound can date a pregnancy to within ±5–7 days, making it more accurate than LMP-based calculation for conception estimation.
Does cycle length affect the due date estimate?
Yes. Each day your average cycle differs from 28 days shifts both the estimated LMP and conception date by roughly one day. A 35-day cycle pushes conception about 7 days later than a 28-day cycle, which also shifts the EDD by the same amount.
What is Naegele's Rule?
Naegele's Rule is the standard formula for estimating due date: add 280 days (40 weeks) to the first day of LMP. It assumes a 28-day cycle with ovulation on day 14. This calculator extends the rule to account for cycle lengths other than 28 days.
Can I use this to determine paternity?
No. This calculator estimates a probable conception window, but sperm can survive in the reproductive tract for up to 5 days, making it impossible to pinpoint a single act of intercourse from dates alone. Legal or medical paternity determination requires DNA testing.
Why does the calculator show gestational age from LMP, not from the conception date?
Because all clinical milestones, prenatal screening windows, and trimester boundaries are defined relative to LMP. Reporting gestational age from conception would require subtracting ~2 weeks from every published reference range, creating confusion. The output matches what your doctor sees.