CalcKitHome›Family & Life›Conception: two possible fathers
Conception Calculator: Two Possible Fathers
Your dates
Estimates only. Only a DNA test can determine paternity.
Example: last period August 3, 2026 (28-day cycle) — ovulation August 17, fertile window August 12–18. August 16 fits the window; August 26 is 9 days after ovulation and unlikely. Due date May 10, 2027.
Behind the numbers
How the two dates are compared
Pregnancy is dated from the first day of the last period, and conception usually happens around ovulation — about 14 days after the period starts in a typical 28-day cycle, or 14 days before the next period if your cycles are longer or shorter. Sperm can live inside the body for up to 5 days, and the egg survives about a day after ovulation, so the fertile window runs from 5 days before ovulation to 1 day after it. Each date you enter is measured against that window: a date inside the window fits, a date just outside it is less likely, and a date well outside it is unlikely. The date closer to ovulation is the more likely of the two — but “more likely” is not proof.
For a last period starting August 3, 2026 with a 28-day cycle: ovulation = August 3 + 14 days = August 17, 2026. The fertile window is ovulation − 5 days to ovulation + 1 day = August 12 to August 18, 2026. The due date = August 3, 2026 + 280 days = May 10, 2027. August 16 is 1 day before ovulation (inside the window); August 26 is 9 days after ovulation (outside it).
If you enter a due date instead, the calculator works backwards: the last period was about 280 days before the due date, and ovulation was about 266 days before it. If you enter an ultrasound date, it subtracts how far along you were at the scan to find the last period, then works forward the same way.
Examples
Example comparisons
| What you know | Date 1 | Date 2 | More likely |
|---|---|---|---|
| Last period Aug 3, 2026 (28-day cycle) | Aug 16, 2026 | Aug 26, 2026 | Date 1 — inside window |
| Last period Mar 2, 2026 (28-day cycle) | Mar 15, 2026 | Mar 17, 2026 | Both inside — dates cannot separate them |
| Due date May 10, 2027 | Aug 16, 2026 | Aug 26, 2026 | Date 1 — inside window |
March 2, 2026 + 14 days = ovulation March 16, window March 11–17: both March 15 and March 17 fall inside it, so only a DNA test can tell them apart.
What the dates can and cannot tell you.
A date that fits the window is possible, not proven: sperm can wait up to 5 days for the egg, so conception may have happened days before ovulation.
If both dates are inside the fertile window, the dates alone cannot tell them apart — that is exactly the situation a DNA test is for.
An early ultrasound (first trimester) dates a pregnancy to within a few days and is far more accurate than counting from the last period, especially with irregular cycles.
If this is a difficult or sensitive situation, talk to your doctor or a counselor — they can explain DNA testing options and what the results mean, without judgment.
Good to know
Your questions, answered.
Can dates prove who the father is?
No. Dates can only show which date fits the pregnancy timing better. Sperm can live for up to 5 days, ovulation day varies, and the due date itself is an estimate — so even a date that fits perfectly is not proof. Only a DNA paternity test can determine who the father is.
What if both dates fall inside the fertile window?
Then the dates cannot separate them: both are possible, and the one closer to ovulation is only slightly more likely. In that case a DNA test is the only way to know for certain.
What if my cycles are irregular or much longer than 28 days?
Then ovulation probably did not happen on day 14. A rough rule is that ovulation happens about 14 days before the next period, not 14 days after the last one — so a 35-day cycle means ovulation around day 21. If your cycles vary, treat the fertile window as wider, and an early ultrasound dates the pregnancy far more accurately.
Is this medical advice?
No. This is a general estimate for planning and information. For dating a pregnancy, paternity questions, or any medical decision, talk to your doctor — only an ultrasound, clinical history and, for paternity, a DNA test can give a reliable answer.