Enter the first day of your last menstrual period (LMP) to estimate your current gestational age and estimated due date.
function calculatePregnancyWeeks() {
var lmpDateInput = document.getElementById("lmpDate").value;
var resultDiv = document.getElementById("pregnancyResult");
if (!lmpDateInput) {
resultDiv.innerHTML = "Please enter the first day of your Last Menstrual Period.";
return;
}
var lmpDate = new Date(lmpDateInput);
var today = new Date();
// Set time to 00:00:00 for accurate day difference calculation
lmpDate.setHours(0, 0, 0, 0);
today.setHours(0, 0, 0, 0);
// Calculate the difference in milliseconds
var timeDiff = today.getTime() – lmpDate.getTime();
// Convert milliseconds to days
var daysPregnant = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
if (daysPregnant < 0) {
resultDiv.innerHTML = "The LMP date cannot be in the future. Please enter a valid date.";
return;
}
var weeksPregnant = Math.floor(daysPregnant / 7);
var remainingDays = daysPregnant % 7;
// Calculate Estimated Due Date (EDD)
// EDD is typically 280 days (40 weeks) from LMP
var dueDate = new Date(lmpDate.getTime());
dueDate.setDate(lmpDate.getDate() + 280);
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedLmpDate = lmpDate.toLocaleDateString('en-US', options);
var formattedDueDate = dueDate.toLocaleDateString('en-US', options);
resultDiv.innerHTML =
"Based on your LMP of " + formattedLmpDate + ":" +
"You are approximately " + weeksPregnant + " weeks and " + remainingDays + " days pregnant." +
"Your Estimated Due Date (EDD) is: " + formattedDueDate + "." +
"Please note: This is an estimate. Always consult your healthcare provider for confirmation.";
}
Understanding Your Pregnancy Weeks
Knowing your gestational age, or how many weeks pregnant you are, is a crucial piece of information throughout your pregnancy journey. It helps healthcare providers monitor your baby's development, schedule important tests and screenings, and estimate your baby's arrival date. This calculator uses the most common method for initial dating: the first day of your Last Menstrual Period (LMP).
How Pregnancy Weeks Are Calculated
Pregnancy is typically considered to last about 40 weeks (or 280 days) from the first day of your last menstrual period. This method assumes that conception occurs around two weeks after your LMP, making your gestational age about two weeks ahead of the actual fetal age. While it might seem counterintuitive to count weeks before conception, this standardized method provides a consistent starting point for all pregnancies.
The Role of Your Last Menstrual Period (LMP)
Your LMP is the cornerstone of this calculation. By inputting the exact date of the first day of your last period, the calculator determines how many days have passed since then. This total number of days is then converted into weeks and remaining days to give you your current gestational age. It also projects your estimated due date by adding 280 days (40 weeks) to your LMP date.
Why is Gestational Age Important?
Monitoring Development: Each week of pregnancy brings specific developmental milestones for your baby. Knowing your gestational age helps your doctor ensure your baby is growing as expected.
Scheduling Tests: Many prenatal tests, such as ultrasounds, genetic screenings, and glucose tolerance tests, are time-sensitive and need to be performed within specific gestational windows.
Planning for Delivery: Your estimated due date helps you and your healthcare team prepare for labor and delivery. While only a small percentage of babies are born exactly on their due date, it provides a valuable target.
Limitations and Considerations
While the LMP method is widely used, it has limitations. It is most accurate for individuals with regular menstrual cycles. If your cycles are irregular, or if you're unsure of your LMP, an early ultrasound (typically between 8 and 12 weeks) can provide a more accurate dating of your pregnancy. Always consult with your healthcare provider for personalized medical advice and confirmation of your gestational age and due date.
Example Calculation
Let's say your Last Menstrual Period (LMP) started on January 1, 2024. If today's date is April 15, 2024:
Days from LMP to Today: January (31-1=30 days remaining) + February (29 days for leap year 2024) + March (31 days) + April (15 days) = 105 days.
Weeks Pregnant: 105 days / 7 days/week = 15 weeks.
Remaining Days: 105 % 7 = 0 days.
Estimated Due Date: January 1, 2024 + 280 days = October 8, 2024.
So, based on these dates, you would be 15 weeks pregnant with an estimated due date of October 8, 2024.