Use this calculator to estimate your due date, conception date, and current week of pregnancy based on the first day of your last menstrual period (LMP).
Understanding Your Pregnancy Dates
A pregnancy calculator is a helpful tool for expectant parents to estimate key milestones throughout their pregnancy journey. While an ultrasound provides the most accurate dating, especially in early pregnancy, a calculator based on your Last Menstrual Period (LMP) offers a good initial estimate.
How Does It Work?
Most pregnancy calculators, including this one, use the first day of your Last Menstrual Period (LMP) as the starting point. Pregnancy is typically considered to last about 40 weeks (280 days) from the first day of your LMP. This method assumes that conception occurred approximately two weeks after your LMP, around the time of ovulation.
Key Dates Explained:
Estimated Due Date (EDD): This is the date your baby is expected to arrive. It's calculated by adding 280 days (40 weeks) to the first day of your LMP. Remember, only about 5% of babies are born exactly on their due date, so it's more of a target window.
Conception Date: This is the approximate date when fertilization likely occurred. It's typically estimated to be about 14 days after the first day of your LMP, assuming a regular 28-day cycle.
Current Week of Pregnancy: This tells you how far along you are in your pregnancy, calculated from your LMP to today's date.
Trimester Dates: Pregnancy is divided into three trimesters, each marking significant developmental stages for the baby and changes for the mother.
First Trimester: Weeks 1-13. This period involves rapid development of major organs.
Second Trimester: Weeks 14-27. Often considered the "golden trimester" as many early pregnancy symptoms subside, and the baby grows significantly.
Third Trimester: Weeks 28-40 (or until birth). The baby continues to grow and mature, preparing for birth.
Example Calculation:
Let's say your Last Menstrual Period (LMP) started on October 26, 2023.
Estimated Due Date: August 2, 2024 (October 26, 2023 + 280 days)
Current Week of Pregnancy: Approximately 13 weeks pregnant.
Current Trimester: First Trimester.
End of First Trimester: Around January 31, 2024.
End of Second Trimester: Around May 8, 2024.
Important Considerations:
While this calculator provides useful estimates, it's important to consult with your healthcare provider for personalized and accurate dating. Factors like irregular menstrual cycles can affect the accuracy of LMP-based calculations. An early ultrasound can often provide a more precise due date.
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function calculatePregnancyDates() {
var lmpDateInput = document.getElementById("lmpDate").value;
var resultsDiv = document.getElementById("pregnancyResults");
resultsDiv.innerHTML = ""; // Clear previous results
if (!lmpDateInput) {
resultsDiv.innerHTML = "Please enter the first day of your Last Menstrual Period.";
return;
}
var lmp = new Date(lmpDateInput);
if (isNaN(lmp.getTime())) {
resultsDiv.innerHTML = "Invalid date entered. Please use a valid date format.";
return;
}
var today = new Date();
today.setHours(0, 0, 0, 0); // Normalize today's date to start of day
// Check if LMP is in the future
if (lmp > today) {
resultsDiv.innerHTML = "The LMP date cannot be in the future. Please enter a past or current date.";
return;
}
// — Calculate Estimated Due Date (EDD) —
var edd = new Date(lmp.getTime());
edd.setDate(edd.getDate() + 280); // 40 weeks = 280 days
// — Calculate Conception Date —
var conceptionDate = new Date(lmp.getTime());
conceptionDate.setDate(conceptionDate.getDate() + 14); // Approximately 2 weeks after LMP
// — Calculate Current Week of Pregnancy —
var diffTime = Math.abs(today.getTime() – lmp.getTime());
var diffDays = Math.ceil(diffTime / (1000 * 60 * 60 * 24));
var currentWeek = Math.floor(diffDays / 7) + 1; // +1 because week 1 starts on day 0-6
// — Determine Current Trimester —
var currentTrimester = "";
if (currentWeek >= 1 && currentWeek = 14 && currentWeek = 28 && currentWeek 40) {
currentTrimester = "Full Term (past due date)";
} else {
currentTrimester = "Not yet pregnant (LMP is too recent)";
}
// — Calculate Trimester End Dates —
var endOfFirstTrimester = new Date(lmp.getTime());
endOfFirstTrimester.setDate(endOfFirstTrimester.getDate() + (13 * 7) + 6); // End of week 13
var endOfSecondTrimester = new Date(lmp.getTime());
endOfSecondTrimester.setDate(endOfSecondTrimester.getDate() + (27 * 7) + 6); // End of week 27
// — Format Dates for Display —
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedLMP = lmp.toLocaleDateString('en-US', options);
var formattedEDD = edd.toLocaleDateString('en-US', options);
var formattedConception = conceptionDate.toLocaleDateString('en-US', options);
var formattedEndOfFirstTrimester = endOfFirstTrimester.toLocaleDateString('en-US', options);
var formattedEndOfSecondTrimester = endOfSecondTrimester.toLocaleDateString('en-US', options);
// — Display Results —
var html = "
Your Pregnancy Estimates:
";
html += "First Day of LMP: " + formattedLMP + "";
html += "Estimated Due Date (EDD): " + formattedEDD + "";
html += "Approximate Conception Date: " + formattedConception + "";
html += "Current Week of Pregnancy: Week " + currentWeek + "";
html += "Current Trimester: " + currentTrimester + "";
if (currentWeek <= 13) {
html += "End of First Trimester: " + formattedEndOfFirstTrimester + "";
html += "End of Second Trimester: " + formattedEndOfSecondTrimester + "";
} else if (currentWeek <= 27) {
html += "End of Second Trimester: " + formattedEndOfSecondTrimester + "";
}
resultsDiv.innerHTML = html;
}
// Optional: Set default LMP date to 10 weeks ago for initial display
document.addEventListener('DOMContentLoaded', function() {
var today = new Date();
var defaultLMP = new Date(today.getTime());
defaultLMP.setDate(defaultLMP.getDate() – (10 * 7)); // 10 weeks ago
var year = defaultLMP.getFullYear();
var month = (defaultLMP.getMonth() + 1).toString().padStart(2, '0');
var day = defaultLMP.getDate().toString().padStart(2, '0');
document.getElementById("lmpDate").value = year + '-' + month + '-' + day;
calculatePregnancyDates(); // Calculate on load with default value
});