Estimate your next period based on your cycle history. If your period is late, this calculator can help you understand your typical cycle length and potential reasons for a delay.
Understanding Your Menstrual Cycle and Period Delays
The menstrual cycle is a natural series of changes a woman's body goes through to prepare for the possibility of pregnancy. Each month, one of the ovaries releases an egg (ovulation). At the same time, hormonal changes prepare the uterus for pregnancy. If ovulation takes place and the egg isn't fertilized by a sperm, the lining of the uterus sheds, resulting in menstrual bleeding (a period).
What is an Average Menstrual Cycle?
A typical menstrual cycle is often considered to be 28 days long, but this is just an average. For many women, a normal cycle can range anywhere from 21 to 35 days. The cycle length is counted from the first day of one period to the first day of the next.
How the Late Menstrual Period Calculator Works:
This calculator uses two key pieces of information to estimate your next expected period:
Date of Last Menstrual Period (LMP) Start: This is the first day you started bleeding for your most recent period.
Average Menstrual Cycle Length: This is the typical number of days between the start of one period and the start of the next. It's best to calculate this from at least 3-6 months of your cycle history.
The calculator adds your average cycle length (in days) to the start date of your last period. The result is the estimated date of your next period's start. By comparing this expected date to "today's date" (or any date you input), you can determine if your period is on time, early, or late.
When to Consider a Period "Late":
A period is generally considered late if it is more than 5 days after your period was expected based on your average cycle length. For example, if your cycle is consistently 28 days and you expect your period on the 15th of the month, a period arriving on the 20th or later might be considered late.
Common Reasons for a Late Period (Besides Pregnancy):
While pregnancy is a common reason for a missed or late period, there are many other factors that can influence your cycle:
Stress: Significant emotional or physical stress can disrupt the hormones that regulate your cycle.
Changes in Weight: Both significant weight loss and weight gain can affect ovulation.
Excessive Exercise: Intense physical activity can sometimes interfere with menstruation.
Medical Conditions: Polycystic Ovary Syndrome (PCOS), thyroid problems, and other hormonal imbalances can cause irregular cycles.
Medications: Certain drugs, including some contraceptives, antidepressants, and antipsychotics, can alter your cycle.
Perimenopause: As women approach menopause, their menstrual cycles can become irregular.
Sleep Schedule Changes: Disruptions to your sleep pattern can affect hormone levels.
When to Seek Medical Advice:
If your period is consistently late, more than a week or two overdue, or if you have other concerning symptoms, it's always a good idea to consult with a healthcare professional. They can help identify any underlying causes and provide appropriate guidance or treatment.
function calculatePeriodDate() {
var lmpStartDateInput = document.getElementById("lastPeriodStartDate");
var avgCycleLengthInput = document.getElementById("avgCycleLength");
var currentDateInput = document.getElementById("currentDate");
var resultDiv = document.getElementById("result");
var lmpStartDateStr = lmpStartDateInput.value;
var avgCycleLengthStr = avgCycleLengthInput.value;
var currentDateStr = currentDateInput.value;
// Clear previous results and classes
resultDiv.innerHTML = "";
resultDiv.className = "";
// Validate inputs
if (!lmpStartDateStr || !avgCycleLengthStr || !currentDateStr) {
resultDiv.innerHTML = "Please fill in all fields.";
return;
}
var avgCycleLength = parseInt(avgCycleLengthStr, 10);
if (isNaN(avgCycleLength) || avgCycleLength <= 0) {
resultDiv.innerHTML = "Please enter a valid average cycle length (a positive number).";
return;
}
// Get dates as Date objects
var lmpStartDate = new Date(lmpStartDateStr);
var currentDate = new Date(currentDateStr);
// Calculate expected period start date
var expectedPeriodStartDate = new Date(lmpStartDate);
expectedPeriodStartDate.setDate(lmpStartDate.getDate() + avgCycleLength);
// Format dates for display
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedExpectedPeriodStartDate = expectedPeriodStartDate.toLocaleDateString(undefined, options);
var formattedCurrentDate = currentDate.toLocaleDateString(undefined, options);
// Compare current date with expected period start date
// Normalize dates to midnight for accurate comparison
var normalizedCurrentDate = new Date(currentDate.getFullYear(), currentDate.getMonth(), currentDate.getDate());
var normalizedExpectedPeriodStartDate = new Date(expectedPeriodStartDate.getFullYear(), expectedPeriodStartDate.getMonth(), expectedPeriodStartDate.getDate());
var timeDiff = normalizedCurrentDate.getTime() – normalizedExpectedPeriodStartDate.getTime();
var daysLate = Math.ceil(timeDiff / (1000 * 60 * 60 * 24));
var message = "";
var resultClass = "";
if (daysLate = 0 && daysLate <= 5) { // Period is on time or up to 5 days late
message = "Your expected period start date was " + formattedExpectedPeriodStartDate + ". Today is " + formattedCurrentDate + ". Your period is currently on time or slightly late.";
resultClass = "on-time";
} else { // Period is more than 5 days late
message = "Your expected period start date was " + formattedExpectedPeriodStartDate + ". Today is " + formattedCurrentDate + ". Your period appears to be late.";
resultClass = "late";
}
resultDiv.innerHTML = message;
resultDiv.className = resultClass;
}