Estimate your current pregnancy week, estimated due date, and trimester based on the first day of your Last Menstrual Period (LMP).
Understanding Your Pregnancy Weeks
A pregnancy is typically dated from the first day of your Last Menstrual Period (LMP), even though conception usually occurs about two weeks later. This method provides a consistent starting point for tracking the 40-week gestation period and is the standard used by healthcare professionals.
This calculator uses the LMP method to estimate your current week of pregnancy, your estimated due date, and which trimester you are currently in. It assumes a regular 28-day menstrual cycle.
How Pregnancy Weeks are Counted:
Week 0-2: Starts from your LMP. Conception typically occurs around week 2.
Week 3-8: Embryonic period, characterized by rapid development of major organs.
Week 9-40+: Fetal period, focusing on growth, maturation, and preparation for birth.
Trimesters Explained:
First Trimester (Weeks 1-13): This period involves significant hormonal changes, common symptoms like morning sickness, and the foundational development of the baby's body systems.
Second Trimester (Weeks 14-27): Often referred to as the "golden trimester," many early symptoms subside, and you might start feeling the baby move. The baby grows rapidly, and its organs continue to mature.
Third Trimester (Weeks 28-40+): The final stage of pregnancy, where the baby gains significant weight, lungs mature, and the body prepares for labor and delivery.
Important Considerations:
While this calculator provides a helpful estimate, it's important to remember that it is not a substitute for professional medical advice. Your healthcare provider will confirm your due date through ultrasound and other assessments, which can sometimes adjust the initial estimate based on fetal measurements.
Factors such as irregular menstrual cycles, unknown LMP, or early ultrasound findings can influence the accuracy of LMP-based dating. Always consult with your doctor or midwife for personalized guidance regarding your pregnancy and due date.
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window.onload = function() {
var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0'); // January is 0!
var yyyy = today.getFullYear();
document.getElementById('todayDate').value = yyyy + '-' + mm + '-' + dd;
// Set LMP date to 9 weeks ago as a common starting point for a user looking for this calculator
var lmpDefault = new Date();
lmpDefault.setDate(today.getDate() – (9 * 7)); // 9 weeks ago
var lmp_dd = String(lmpDefault.getDate()).padStart(2, '0');
var lmp_mm = String(lmpDefault.getMonth() + 1).padStart(2, '0');
var lmp_yyyy = lmpDefault.getFullYear();
document.getElementById('lmpDate').value = lmp_yyyy + '-' + lmp_mm + '-' + lmp_dd;
};
function calculatePregnancyWeek() {
var lmpDateStr = document.getElementById("lmpDate").value;
var todayDateStr = document.getElementById("todayDate").value;
var resultDiv = document.getElementById("result");
if (!lmpDateStr || !todayDateStr) {
resultDiv.innerHTML = "Please enter both the First Day of LMP and Today's Date.";
return;
}
var lmpDate = new Date(lmpDateStr);
var todayDate = new Date(todayDateStr);
// Reset time components to avoid issues with timezones/daylight saving affecting day count
lmpDate.setHours(0, 0, 0, 0);
todayDate.setHours(0, 0, 0, 0);
if (todayDate < lmpDate) {
resultDiv.innerHTML = "Today's Date cannot be before the First Day of LMP. Please check your dates.";
return;
}
var diffTime = Math.abs(todayDate.getTime() – lmpDate.getTime());
var diffDays = Math.floor(diffTime / (1000 * 60 * 60 * 24)); // Number of full days passed
var currentPregnancyWeek = Math.floor(diffDays / 7);
var daysIntoWeek = diffDays % 7;
// Estimated Due Date (EDD) – Add 280 days (40 weeks) to LMP
var estimatedDueDate = new Date(lmpDate.getTime());
estimatedDueDate.setDate(lmpDate.getDate() + 280);
var edd_dd = String(estimatedDueDate.getDate()).padStart(2, '0');
var edd_mm = String(estimatedDueDate.getMonth() + 1).padStart(2, '0');
var edd_yyyy = estimatedDueDate.getFullYear();
var formattedDueDate = edd_mm + '/' + edd_dd + '/' + edd_yyyy;
// Weeks remaining
var weeksRemaining = 40 – currentPregnancyWeek;
if (weeksRemaining = 1 && currentPregnancyWeek = 14 && currentPregnancyWeek = 28 && currentPregnancyWeek 40) {
trimester = "Post-Term (beyond 40 weeks)";
} else {
trimester = "Not yet in a recognized trimester";
}
var resultHTML = "
Your Pregnancy Details:
";
if (currentPregnancyWeek === 0) {
resultHTML += "It looks like you are in Week 0 of pregnancy, meaning less than one full week has passed since your LMP. Conception typically occurs around Week 2.";
resultHTML += "Days since LMP: " + diffDays + " day(s)";
} else {
resultHTML += "Current Pregnancy Week: Week " + currentPregnancyWeek + " and " + daysIntoWeek + " day(s)";
}
resultHTML += "Estimated Due Date: " + formattedDueDate + "";
resultHTML += "Current Trimester: " + trimester + "";
resultHTML += "Weeks Remaining until Due Date: " + weeksRemaining + " weeks";
if (currentPregnancyWeek > 42) {
resultHTML += "You are significantly past your estimated due date. Please consult your healthcare provider for guidance.";
}
resultDiv.innerHTML = resultHTML;
}