Enter the first day of your last menstrual period (LMP) to estimate your current gestational age and due date.
Understanding Your Pregnancy Weeks
Knowing how many weeks pregnant you are is crucial for tracking your baby's development, scheduling prenatal appointments, and preparing for delivery. This calculator uses the standard method of counting pregnancy from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later. This method assumes a 28-day menstrual cycle and a 40-week (280-day) pregnancy.
How the Calculator Works
Our "How Many Weeks Pregnant Calculator" takes the date you provide as the first day of your Last Menstrual Period (LMP). It then calculates the number of days that have passed between your LMP and today's date. This total number of days is then converted into weeks and days to give you your current gestational age. Additionally, it estimates your due date by adding 280 days (40 weeks) to your LMP date.
Why LMP is Used
The LMP is used because it's often the most reliable date a woman knows. While conception happens later, medical professionals use the LMP as the starting point for pregnancy dating. This helps standardize gestational age tracking across all pregnancies.
Important Considerations
Accuracy: This calculator provides an estimate. The actual due date can vary, and only a healthcare professional can provide a definitive assessment, often confirmed by ultrasound.
Irregular Cycles: If you have irregular menstrual cycles, the LMP method might be less accurate. In such cases, an early ultrasound is usually recommended to determine gestational age more precisely.
40 Weeks: A full-term pregnancy is typically considered 40 weeks from the LMP, though babies can be born healthy anywhere between 37 and 42 weeks.
Example Calculation
Let's say your Last Menstrual Period (LMP) started on January 1, 2023. If today's date is September 1, 2023:
The calculator determines that 243 days have passed since your LMP.
Dividing 243 by 7 gives you 34 with a remainder of 5.
Therefore, you would be approximately 34 weeks and 5 days pregnant.
Your estimated due date would be 280 days after January 1, 2023, which is October 8, 2023.
Always consult with your doctor or midwife for personalized medical advice regarding your pregnancy.
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function calculatePregnancy() {
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.";
resultDiv.classList.add("show");
return;
}
var lmpDate = new Date(lmpDateInput);
var today = new Date();
// Normalize today's date to midnight for accurate day calculation
today.setHours(0, 0, 0, 0);
lmpDate.setHours(0, 0, 0, 0);
if (isNaN(lmpDate.getTime())) {
resultDiv.innerHTML = "Please enter a valid date for your Last Menstrual Period.";
resultDiv.classList.add("show");
return;
}
if (lmpDate > today) {
resultDiv.innerHTML = "Your LMP date cannot be in the future.";
resultDiv.classList.add("show");
return;
}
// Calculate difference in milliseconds
var timeDiff = today.getTime() – lmpDate.getTime();
// Convert to days
var daysDiff = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
if (daysDiff < 0) {
resultDiv.innerHTML = "An error occurred. Please check your LMP date.";
resultDiv.classList.add("show");
return;
}
var weeks = Math.floor(daysDiff / 7);
var days = daysDiff % 7;
// Calculate Estimated Due Date (EDD)
var eddDate = new Date(lmpDate.getTime());
eddDate.setDate(lmpDate.getDate() + 280); // Add 280 days (40 weeks)
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedEdd = eddDate.toLocaleDateString('en-US', options);
resultDiv.innerHTML =
"You are approximately " + weeks + " weeks and " + days + " days pregnant." +
"Your estimated due date is " + formattedEdd + ".";
resultDiv.classList.add("show");
}