Calculate your estimated due date based on the first day of your last menstrual period (LMP).
Understanding Your Estimated Due Date
Pregnancy is an incredible journey, and for many expectant parents, knowing the estimated due date (EDD) is a significant milestone. The American Pregnancy Association's due date calculator provides a simple yet effective way to estimate when your baby might arrive. The most common method for calculating this date is based on the first day of your last menstrual period (LMP).
How the Calculation Works: Naegele's Rule
The calculator uses a widely accepted method known as Naegele's Rule. This rule is based on the assumption that a typical pregnancy lasts 40 weeks (280 days) from the first day of the LMP. The rule is applied as follows:
Step 1: Take the first day of your Last Menstrual Period (LMP).
Step 2: Add 7 days to this date.
Step 3: Subtract 3 months from the resulting date.
Step 4: Add 1 year to the resulting date.
Alternatively, and as implemented in this calculator, it's equivalent to:
Step 1: Take the first day of your LMP.
Step 2: Add 7 days.
Step 3: Add 9 months (or 280 days) to the original LMP date.
This calculation provides an estimated due date. It's important to remember that only about 5% of babies are born exactly on their due date. Full-term pregnancy is generally considered to be anywhere between 37 and 42 weeks.
Why is LMP Used?
The LMP is often used because it's a reliable starting point for dating a pregnancy. Ovulation typically occurs about two weeks after the start of the LMP, and conception follows shortly after. Therefore, LMP is a convenient and consistent marker to begin the 40-week gestation countdown.
What if Your Cycles Aren't Regular?
Naegele's Rule works best for individuals with regular menstrual cycles of approximately 28 days. If your cycles are significantly shorter or longer, or if you have irregular periods, your estimated due date might be less accurate. In such cases, your healthcare provider may use other methods, such as an early ultrasound, to get a more precise estimate of your baby's gestational age and due date.
Consult Your Healthcare Provider
While this calculator is a helpful tool, it is not a substitute for professional medical advice. Always consult with your doctor or midwife regarding your pregnancy. They will confirm your due date, monitor your baby's growth, and provide the best guidance for a healthy pregnancy and delivery.
Disclaimer: This calculator is for informational purposes only and is based on the American Pregnancy Association guidelines. Consult your healthcare provider for personalized medical advice.
function calculateDueDate() {
var lmpDateInput = document.getElementById("lmpDate");
var resultDiv = document.getElementById("result");
if (!lmpDateInput.value) {
resultDiv.innerHTML = "Please enter the first day of your last menstrual period.";
resultDiv.style.backgroundColor = "#ffc107"; /* Warning yellow */
return;
}
var lmpDate = new Date(lmpDateInput.value);
// Validate if the entered date is a valid date object
if (isNaN(lmpDate.getTime())) {
resultDiv.innerHTML = "Invalid date format. Please use MM/DD/YYYY or a similar valid format.";
resultDiv.style.backgroundColor = "#dc3545"; /* Error red */
return;
}
// Naegele's Rule: LMP + 7 days – 3 months + 1 year OR LMP + 280 days
// Using LMP + 280 days is more straightforward for JavaScript Date objects.
var dueDate = new Date(lmpDate);
dueDate.setDate(lmpDate.getDate() + 280); // Add 280 days
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedDueDate = dueDate.toLocaleDateString(undefined, options);
resultDiv.innerHTML = "Your Estimated Due Date is: " + formattedDueDate;
resultDiv.style.backgroundColor = getComputedStyle(document.documentElement).getPropertyValue('–success-green'); /* Reset to success green */
}