Estimate the exact date of fertilization based on your cycle or due date.
Last Period Date
Known Due Date
Estimated Fertilization Date:
How is the Fertilization Date Calculated?
Fertilization, or conception, occurs when a sperm cell meets an egg. This typically happens within a very narrow window following ovulation. Because most people do not know the exact moment they ovulate, we use clinical formulas to estimate the date.
Method 1: The Last Menstrual Period (LMP) Method
This is the most common method. If you have a regular 28-day cycle, ovulation usually occurs on day 14. If your cycle is longer or shorter, the timing of ovulation shifts. The formula used is: Fertilization Date = (Date of LMP) + (Cycle Length – 14 days)
Method 2: The Due Date Method
A full-term pregnancy is considered to be 40 weeks (280 days) from the last period, but the biological age of the fetus is actually 38 weeks (266 days). If you already have a due date from your doctor, we count back 266 days to find when fertilization likely occurred.
Realistic Examples
Example A: If your last period started on January 1st and you have a 28-day cycle, your fertilization date was likely January 15th.
Example B: If your last period started on January 1st but you have a 32-day cycle, ovulation (and fertilization) likely occurred around January 19th (1st + 18 days).
Example C: If your doctor says your due date is October 20th, counting back 266 days puts your fertilization date around January 27th.
Accuracy Factors
It is important to remember that these are estimates. Factors that can affect the date include:
Cycle Irregularity: Stress, diet, or health conditions can delay ovulation.
Sperm Longevity: Sperm can live inside the female reproductive tract for up to 5 days. You may have had intercourse on Monday, but fertilization didn't occur until Wednesday.
Implantation Timing: It takes 6–12 days after fertilization for the egg to implant in the uterus.
function toggleMethod() {
var isLMP = document.getElementById("useLMP").checked;
document.getElementById("sectionLMP").style.display = isLMP ? "grid" : "none";
document.getElementById("sectionDue").style.display = isLMP ? "none" : "grid";
}
function calculateFertilization() {
var isLMP = document.getElementById("useLMP").checked;
var resultBox = document.getElementById("fertility-result-box");
var mainResult = document.getElementById("mainResult");
var resultDetails = document.getElementById("resultDetails");
var fertilizationDate = new Date();
if (isLMP) {
var lmpVal = document.getElementById("lastPeriodDate").value;
var cycleVal = parseInt(document.getElementById("cycleLength").value);
if (!lmpVal || isNaN(cycleVal)) {
alert("Please enter a valid date and cycle length.");
return;
}
var lmpDate = new Date(lmpVal);
// Ovulation is usually (Cycle Length – 14) days after LMP
var daysToAdd = cycleVal – 14;
fertilizationDate.setTime(lmpDate.getTime() + (daysToAdd * 24 * 60 * 60 * 1000));
resultDetails.innerHTML = "Based on a " + cycleVal + "-day cycle, fertilization typically occurs " + daysToAdd + " days after your period starts.";
} else {
var dueVal = document.getElementById("babyDueDate").value;
if (!dueVal) {
alert("Please enter a valid due date.");
return;
}
var dueDate = new Date(dueVal);
// Subtract 266 days (38 weeks) from due date
fertilizationDate.setTime(dueDate.getTime() – (266 * 24 * 60 * 60 * 1000));
resultDetails.innerHTML = "Calculated by subtracting 266 days (the average length of human gestation from conception) from your due date.";
}
var options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' };
mainResult.innerHTML = fertilizationDate.toLocaleDateString(undefined, options);
resultBox.style.display = "block";
// Scroll to result
resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}