Pregnancy Conception Calculator
Estimate your likely conception date and fertile window from your due date, last period, or ultrasound.
Estimated Conception Date
May 10, 2026
Summary
Working back from a last period date of April 26, 2026, conception most likely occurred around May 10, 2026 — with a fertile window spanning May 5 to May 11 due to sperm survival of up to 5 days.
Fertile Window
May 5 – May 11, 2026
Likely Implantation
May 19, 2026
Timeline from last period
What is a Pregnancy Conception Calculator?
A pregnancy conception calculator works backward from a known due date, last menstrual period, or ultrasound to estimate when conception most likely occurred, along with a fertile window of days during which intercourse could plausibly have led to conception. This is the reverse problem of the standard Pregnancy Calculator, which projects a due date forward from a known start point.
Conception and fertilization aren't quite the same thing medically: fertilization is the moment sperm and egg unite in the fallopian tube, while conception (in the strict medical sense) is only complete once the fertilized egg implants in the uterus, about 6–10 days later. A person is medically considered pregnant only after implantation.
Your Conception Timeline
| Event | Date | Days After LMP |
|---|---|---|
| Last Period Began | Apr 26, 2026 | 0 |
| Fertile Window Starts | May 5, 2026 | 9 |
| Likely Conception | May 10, 2026 | 14 |
| Fertile Window Ends | May 11, 2026 | 15 |
| Likely Implantation | May 19, 2026 | 23 |
| Estimated Due Date | Jan 31, 2027 | 280 |
How the Conception Window Is Estimated
Ovulation — and therefore conception — typically occurs about 14 days before the next expected period, which works out to roughly 11–21 days after the first day of the last period for most cycle lengths. Because sperm can survive in the reproductive tract for up to 5 days while an egg is only viable for about 24 hours after ovulation, the "fertile window" spans the several days before ovulation through about a day after it.
Fertile Window ≈ Conception Date − 5 days to Conception Date + 1 day
Why This Is an Estimate, Not a Certainty
Ovulation timing varies from cycle to cycle even in the same person, and standard calculations assume a consistent luteal phase (the time between ovulation and the next period) of about 14 days — a reasonable population average, but not exact for everyone. Stress, illness, travel, and irregular cycles can all shift ovulation earlier or later than the calculated estimate, sometimes by a week or more.
When This Calculator Is Used
Beyond general curiosity, conception-date estimates are sometimes used in legal contexts (establishing a presumed conception window for paternity questions) and in clinical contexts (dating a pregnancy when the last period date is uncertain or cycles are irregular). In clinical settings, an early ultrasound is considered far more reliable than a last-period-based estimate for these purposes.
Conception vs. Fertilization vs. Implantation
These three terms are often used loosely but describe different biological events. Fertilization happens first, when sperm penetrates the egg in the fallopian tube. The fertilized egg then travels to the uterus over several days before implanting in the uterine lining — implantation — around 6 to 10 days after fertilization. Home pregnancy tests detect the hCG hormone that rises only after implantation, which is why they can't detect pregnancy immediately after intercourse.
Example — Your Current Inputs
Working back from a last period date of April 26, 2026, conception most likely occurred around May 10, 2026 — with a fertile window spanning May 5 to May 11 due to sperm survival of up to 5 days.
Additional Example — Working Back From a Due Date
A doctor confirms a due date of October 10 based on an early ultrasound. Subtracting the standard 280-day pregnancy length places the last menstrual period around January 4, and adding the typical 14-day ovulation offset estimates conception around January 18 — with a fertile window of roughly January 13 to January 19 once sperm and egg viability are factored in.
About These Parameters
- Last Period Date & Cycle Length
- The first day of the last menstrual period before pregnancy, plus the typical number of days between periods. A longer-than-28-day cycle pushes the estimated ovulation (and conception) date later.
- Due Date
- A confirmed or estimated due date, typically from a doctor's ultrasound dating. The calculator subtracts the standard 280-day pregnancy length to reconstruct the implied last-period date before estimating conception.
- Ultrasound Date & Gestational Age at Scan
- The date of an ultrasound and the gestational age measured at that scan. Early first-trimester ultrasounds are the most reliable way to reconstruct an accurate last-period date when actual dates are uncertain.
Frequently Asked Questions
How accurate is a conception date estimate?
It's an estimate with a realistic margin of error of several days to about a week, since it depends on assumptions about ovulation timing that vary between cycles and individuals. An early ultrasound tightens the estimate considerably compared to last-period-only calculations.
Why is the fertile window several days, not just one day?
Because sperm can survive in the reproductive tract for up to 5 days waiting for ovulation, while the egg itself is only viable for about 12–24 hours after being released. Intercourse any time in that combined window can plausibly result in conception, not just intercourse on the exact day of ovulation.
Can this calculator be used for legal paternity questions?
It can provide a general estimate, but courts and medical/legal professionals typically rely on more precise methods (early ultrasound dating or genetic testing) rather than a date-offset calculation alone, especially given the inherent several-day uncertainty in any calculation-based estimate.
Does irregular cycle length affect this estimate?
Yes, significantly — the last-period method assumes a consistent luteal phase length, which is unreliable for irregular cycles. If your cycles vary a lot month to month, an ultrasound-based or due-date-based estimate will be considerably more reliable than the last-period method alone.