Estimating the gestational age of a pregnancy is a crucial part of prenatal care. The most common method used by healthcare professionals is based on the "Naegele's rule," which calculates the duration of pregnancy from the first day of the Last Menstrual Period (LMP). Our calculator simplifies this process for you.
How it Works: The Math Behind the Calculation
Gestational age is typically calculated in weeks and days. The standard convention counts the first day of your last menstrual period as day 1 of the pregnancy. While conception usually occurs about two weeks after the LMP, the dating still starts from the LMP.
The formula is straightforward:
Determine the number of days between the first day of your Last Menstrual Period (LMP) and the current date.
Divide the total number of days by 7 to get the number of full weeks.
The remainder of the division (if any) represents the number of additional days.
For example, if your LMP was on January 1st and today's date is January 15th:
Number of days = 15 – 1 = 14 days.
Weeks pregnant = 14 days / 7 days/week = 2 weeks.
Remaining days = 0.
So, you are 2 weeks and 0 days pregnant.
If your LMP was on January 1st and today's date is January 18th:
Number of days = 18 – 1 = 17 days.
Weeks pregnant = 17 days / 7 days/week = 2 with a remainder of 3.
So, you are 2 weeks and 3 days pregnant.
Why Use This Calculator?
Early Estimation: Get a quick estimate of your pregnancy duration without needing complex manual calculations.
Planning: Helps in understanding developmental milestones, preparing for appointments, and anticipating due dates.
Confirmation: While this calculator provides an estimate, it's always best to confirm your gestational age with a healthcare provider, who may use ultrasound or other methods for more precise dating.
Important Considerations:
This calculator assumes a regular menstrual cycle. If your cycles are irregular, the estimated gestational age based on LMP may be less accurate.
The due date is typically estimated at 40 weeks from the first day of your LMP.
function calculateWeeksPregnant() {
var lmpInput = document.getElementById("lastPeriodStart");
var currentDateInput = document.getElementById("currentDate");
var resultDiv = document.getElementById("result");
var lmpValue = lmpInput.value;
var currentDateValue = currentDateInput.value;
if (!lmpValue || !currentDateValue) {
resultDiv.textContent = "Please select both dates.";
return;
}
var lmpDate = new Date(lmpValue);
var currentDate = new Date(currentDateValue);
// Check if dates are valid
if (isNaN(lmpDate.getTime()) || isNaN(currentDate.getTime())) {
resultDiv.textContent = "Invalid date format. Please use YYYY-MM-DD.";
return;
}
// Ensure current date is not before LMP
if (currentDate < lmpDate) {
resultDiv.textContent = "Today's date cannot be before your last period start date.";
return;
}
// Calculate the difference in milliseconds
var timeDiff = currentDate.getTime() – lmpDate.getTime();
// Convert milliseconds to days
var daysDiff = Math.ceil(timeDiff / (1000 * 3600 * 24));
// Calculate weeks and remaining days
var weeks = Math.floor(daysDiff / 7);
var remainingDays = daysDiff % 7;
resultDiv.textContent = weeks + " weeks and " + remainingDays + " days";
}