The probability of becoming pregnant depends heavily on the "fertile window." Biologically, a woman is only fertile for a few days each month. This window typically includes the five days leading up to ovulation and the day of ovulation itself.
Our calculator uses the Standard Days Method combined with statistical data on age-related fertility and contraceptive failure rates. If your cycle is 28 days, ovulation usually occurs around day 14. Sperm can survive inside the female reproductive tract for up to 5 days, while the egg only survives for 12 to 24 hours after release.
Conception Odds by Cycle Day
Days Relative to Ovulation
Conception Probability (Approx.)
5 Days Before
10%
2 Days Before
25% – 30%
Day of Ovulation
33%
1 Day After
0% – 5%
Factors That Influence Your Results
Cycle Regularity: If your cycles vary in length, the exact day of ovulation is harder to predict.
Age: Fertility naturally declines as women age, particularly after 35, due to lower egg quantity and quality.
Contraception: Even with perfect use, most forms of birth control have a small failure rate. "Typical use" (as used in this calculator) accounts for human error.
Sperm Health: This calculator assumes average male fertility. Factors like sperm count and motility also play a significant role.
When to Take a Pregnancy Test
Regardless of the calculated probability, the most accurate time to take a pregnancy test is at least one day after your period was supposed to start. If you have an irregular cycle, wait at least 21 days after unprotected intercourse for a reliable result. Modern "Early Detection" tests can sometimes detect pregnancy 4-5 days before your expected period, but the risk of a false negative is higher.
function calculatePregnancyChance() {
var cycleLength = parseFloat(document.getElementById('cycleLength').value);
var intercourseDay = parseFloat(document.getElementById('intercourseDay').value);
var ageMultiplier = parseFloat(document.getElementById('ageRange').value);
var contraMultiplier = parseFloat(document.getElementById('contraception').value);
var resultBox = document.getElementById('result-box');
var resultValue = document.getElementById('result-value');
var resultDesc = document.getElementById('result-desc');
if (isNaN(cycleLength) || isNaN(intercourseDay)) {
alert("Please enter valid numbers for cycle length and day of intercourse.");
return;
}
// Estimate ovulation day (Luteal phase is usually 14 days)
var ovulationDay = cycleLength – 14;
var diff = intercourseDay – ovulationDay;
var baseProb = 0;
// Fertility window logic based on clinical data
if (diff === 0) {
baseProb = 0.33; // Day of ovulation
} else if (diff === -1) {
baseProb = 0.31; // 1 day before
} else if (diff === -2) {
baseProb = 0.27; // 2 days before
} else if (diff === -3) {
baseProb = 0.15; // 3 days before
} else if (diff === -4) {
baseProb = 0.10; // 4 days before
} else if (diff === -5) {
baseProb = 0.05; // 5 days before
} else if (diff === 1) {
baseProb = 0.08; // 1 day after (possible overlap)
} else if (diff > 1 && diff 15) {
resultBox.className = "result-high";
resultDesc.innerHTML = "You are currently in a high-fertility window. If you are not trying to conceive, consider emergency contraception if appropriate.";
} else if (finalChance > 2) {
resultBox.className = "result-high";
resultDesc.innerHTML = "There is a moderate chance of conception. This timing is within the extended fertile window.";
} else {
resultBox.className = "result-low";
resultDesc.innerHTML = "Statistically, the chance of pregnancy from this specific instance is low, but never zero if unprotected intercourse occurred.";
}
}