Determine the statistical likelihood of paternity by comparing intercourse dates against your estimated fertile window. This clinical tool utilizes reproductive algorithms regarding sperm viability (up to 5 days) and ovulation timing to calculate which partner’s timing aligns best with conception. While only a DNA test can provide 100% certainty, this calculator provides a comparative probability assessment based on your specific cycle data.
How to use this calculator
Select your dating method
The accuracy of this tool depends on knowing when you ovulated. The most accurate method is an Ultrasound Date from the first trimester (measuring the Crown Rump Length). If you don't have that, use your Last Period (LMP), but ensure your "Average Cycle Length" is accurate.
Enter dates for both partners
Input all intercourse dates that occurred roughly 2 weeks before and 1 week after your estimated conception timing. You can add as many dates as needed for "Partner A" and "Partner B."
Interpret the Probability Score
The calculator compares the dates against your fertile window (the 5 days leading up to ovulation + ovulation day).
How scoring works:
• Intercourse 1-2 days before ovulation scores highest (Peak Fertility).
• Intercourse 4-5 days before scores lower (sperm survival is less likely).
• Intercourse 24+ hours after ovulation scores near zero (the egg dissolves quickly).
Clinical Methodology
This calculator utilizes a probability scoring algorithm based on established reproductive physiology data regarding sperm survival and the "fertile window."
1. The Fertile Window Calculation
Conception is only biologically possible during a specific window of the menstrual cycle, defined by the survival time of gametes:
- Sperm Viability: Sperm can survive in the female reproductive tract for up to 5 days in optimal cervical mucus.
- Egg Viability: The oocyte (egg) survives for only 12 to 24 hours after ovulation.
Therefore, the calculator defines the "Fertile Window" as the 5 days preceding ovulation plus the day of ovulation itself.
2. Probability Scoring Weights
Not all days in the window carry equal weight. Based on clinical studies (Wilcox et al.), the probability of conception is weighted as follows in our algorithm:
- Peak Fertility (High Score): Intercourse occurring 1 to 2 days before ovulation. This ensures a high count of viable sperm are present in the fallopian tubes when the egg is released.
- Moderate Fertility: Intercourse on the day of ovulation or 3 days prior.
- Low Fertility: Intercourse 4–5 days prior (sperm death rate is high) or 1 day after ovulation (egg is likely degenerating).
3. Ovulation Estimation
The accuracy of the result depends entirely on the accuracy of the ovulation date estimation:
- Ultrasound Mode: Uses the Crown-Rump Length (CRL) or Gestational Age from an early scan to back-calculate conception. This is considered the gold standard for dating.
- LMP Mode: Uses the Naegele’s Rule adaptation (LMP + Cycle Length - 14 days). This assumes a consistent luteal phase of 14 days, which varies among individuals.
Medical disclaimer
The information and calculators on this site are educational tools only. They provide statistical estimates based on published research and the details you enter.
They cannot diagnose, predict what will happen for you, or replace personalized advice from a licensed health care professional who knows your full history. Always talk with your doctor, midwife, or other qualified clinician before making decisions about your health, fertility, or pregnancy.
Never ignore, avoid, or delay seeking professional medical advice because of something you read here or a result you see in a calculator. If you think you may be having a medical emergency, call 911 in the United States or your local emergency number.