function calculatePregnancyWeeks() {
var lmpDateInput = document.getElementById("lmpDate").value;
if (!lmpDateInput) {
document.getElementById("result").innerHTML = "Please enter the first day of your Last Menstrual Period (LMP).";
return;
}
var lmpDate = new Date(lmpDateInput);
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to start of day for accurate day calculation
// Check if LMP date is valid
if (isNaN(lmpDate.getTime())) {
document.getElementById("result").innerHTML = "Please enter a valid date for your Last Menstrual Period.";
return;
}
// Check if LMP date is in the future
if (lmpDate > today) {
document.getElementById("result").innerHTML = "Your Last Menstrual Period date cannot be in the future. Please enter a past or current date.";
return;
}
var timeDiff = today.getTime() – lmpDate.getTime();
var daysSinceLMP = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
var currentPregnancyWeek = Math.floor(daysSinceLMP / 7) + 1;
var daysIntoCurrentWeek = daysSinceLMP % 7;
// Calculate Estimated Due Date (EDD) using Naegele's Rule (LMP + 280 days)
var edd = new Date(lmpDate);
edd.setDate(lmpDate.getDate() + 280);
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedLmp = lmpDate.toLocaleDateString('en-US', options);
var formattedEdd = edd.toLocaleDateString('en-US', options);
var resultHtml = "";
if (currentPregnancyWeek > 42) { // Beyond typical full-term + 2 weeks
resultHtml = "Based on your LMP of " + formattedLmp + ", your estimated due date was " + formattedEdd + ". It appears your baby may have already arrived!";
} else {
resultHtml = "Based on your Last Menstrual Period (LMP) of " + formattedLmp + ":";
resultHtml += "You are approximately Week " + currentPregnancyWeek + " of pregnancy, " + daysIntoCurrentWeek + " day(s) into this week.";
resultHtml += "Your Estimated Due Date (EDD) is: " + formattedEdd + ".";
var daysUntilDue = Math.ceil((edd.getTime() – today.getTime()) / (1000 * 60 * 60 * 24));
if (daysUntilDue > 0) {
resultHtml += "That's approximately " + daysUntilDue + " days until your due date.";
} else if (daysUntilDue === 0) {
resultHtml += "Congratulations! Your estimated due date is today!";
} else { // daysUntilDue is negative, meaning EDD has passed
resultHtml += "Your estimated due date was " + formattedEdd + ". Your baby may have already arrived!";
}
}
document.getElementById("result").innerHTML = resultHtml;
}
Understanding Your Pregnancy Week by Week
Congratulations on your pregnancy journey! Knowing which week of pregnancy you are in is crucial for tracking your baby's development, understanding what changes to expect in your body, and preparing for upcoming appointments. Our Pregnancy Week by Week Calculator helps you pinpoint your current stage and estimate your baby's arrival date.
How Pregnancy Weeks Are Calculated
Pregnancy is typically counted as 40 weeks (or 280 days) from the first day of your Last Menstrual Period (LMP). This method is standard even though conception usually occurs about two weeks after your LMP. Doctors use the LMP because it's often the most reliable date for dating a pregnancy, especially in the early stages. This calculation method is commonly known as Naegele's Rule.
First Trimester: Weeks 1-13
Second Trimester: Weeks 14-27
Third Trimester: Weeks 28-40 (or until birth)
Using the Calculator
To use our calculator, simply enter the first day of your Last Menstrual Period (LMP) into the designated field. Click "Calculate My Weeks," and the tool will instantly provide:
Your current pregnancy week and days into that week.
Your Estimated Due Date (EDD).
The approximate number of days remaining until your due date.
The Importance of Your Estimated Due Date (EDD)
Your Estimated Due Date (EDD) is a prediction of when your baby will arrive. While only about 5% of babies are born exactly on their due date, it provides a valuable timeframe for medical professionals to monitor your pregnancy and for you to prepare for childbirth. Your doctor may adjust your EDD based on early ultrasound scans, which can offer a more precise dating, especially if your menstrual cycles are irregular or if there's a significant discrepancy with the LMP dating.
Example Calculation:
Let's say your Last Menstrual Period (LMP) started on January 1, 2024. If today's date is April 15, 2024:
The calculator would determine that approximately 105 days have passed since your LMP.
Dividing 105 by 7 gives 15. So, you would be in your 16th week of pregnancy (15 full weeks + 0 days into the 16th week, as 105/7 = 15 exactly).
Your Estimated Due Date (EDD) would be around October 8, 2024 (January 1, 2024 + 280 days).
The days remaining until your due date would be approximately 176 days.
This calculator is a helpful tool for general guidance. Always consult with your healthcare provider for personalized medical advice and accurate pregnancy dating.