Calculator Input Form
Example Data Table
| Scenario | Cycle Day | Exposure | Emergency Method | Estimated Band |
|---|---|---|---|---|
| Three days before ovulation | Day 11 of 28 | Unprotected | None | Elevated |
| One day before ovulation with condom failure | Day 13 of 28 | Condom failed | Levonorgestrel | Moderate |
| Four days after ovulation with intact condom | Day 18 of 28 | Condom intact | None | Very Low |
Formula Used
The calculator uses a weighted timing model rather than a diagnosis formula. First, it estimates ovulation:
Estimated Ovulation Day = Cycle Length − Luteal Phase Length
Then it locates the intercourse day within the cycle:
Cycle Day of Intercourse = (Intercourse Date − Period Start Date) + 1
After that, the final estimate is built with weighted multipliers:
Likelihood % = Base Chance × Timing Factor × Exposure Factor × Emergency Factor × Ovulation Evidence Factor × Repeat Exposure Factor × Age Factor × Regularity Factor
The base chance is capped and the result is limited to a practical range. Timing carries the strongest weight, especially from five days before ovulation through one day after.
How to Use This Calculator
- Enter the first day of your last period.
- Enter the date of the intercourse event you want assessed.
- Provide your usual cycle length and luteal phase length.
- Select the exposure type and any emergency contraception used.
- Add ovulation clues such as mucus or a positive LH test.
- Choose whether additional intercourse happened in the fertile window.
- Press Calculate Likelihood to see the result above the form.
- Review the graph, detailed metrics, and suggested test date.
Frequently Asked Questions
1. Can this calculator confirm pregnancy?
No. It only estimates relative likelihood from timing and exposure details. Pregnancy can only be confirmed reliably by a urine test, blood test, or clinical evaluation.
2. Why is ovulation timing so important?
Pregnancy likelihood is highest near ovulation because sperm can survive several days, but the egg is available for a much shorter time. Timing changes the score more than most other entries.
3. Does a regular cycle make the estimate better?
Yes. More regular cycles make ovulation timing easier to estimate, so the model gives slightly stronger alignment weighting. Irregular cycles lower confidence in the fertile window estimate.
4. What if I used emergency contraception?
The calculator applies a protective reduction based on the method selected. It still does not replace clinical advice, especially if timing was close to ovulation or your period is late.
5. Does withdrawal count as zero risk?
No. Withdrawal lowers exposure compared with unprotected ejaculation, but it does not remove risk entirely. Timing near ovulation can still produce a nontrivial estimate.
6. When should I take a pregnancy test?
Many people get more useful results around 14 days after intercourse or after a missed period. Very early testing can miss pregnancy even when conception occurred.
7. Why does age appear in the calculation?
Age is used only as a broad fertility weighting. It does not predict your personal fertility, reproductive history, or medical conditions, which may differ substantially from population averages.
8. Should I rely on this tool for contraception decisions?
No. This estimator is educational only. It should never replace consistent contraception use, emergency care, or personalized advice from a qualified medical professional.