Use this calculator to estimate your current pregnancy week, estimated due date, and trimester based on the first day of your last menstrual period (LMP).
Understanding Your Pregnancy Week by Week
A pregnancy calculator is a valuable tool for expectant parents to track their journey from conception to birth. While it provides estimates, it helps in understanding the timeline of pregnancy, key developmental milestones, and when to expect important appointments.
How Pregnancy Weeks Are Calculated
Pregnancy is typically considered to last 40 weeks (280 days) from the first day of your Last Menstrual Period (LMP). This method is used even though conception usually occurs about two weeks after your LMP. This standardized approach helps healthcare providers and expectant parents align on a common timeline.
Last Menstrual Period (LMP): This is the most common starting point for calculating pregnancy duration. It's the first day of your last period before you became pregnant.
Estimated Due Date (EDD): Your EDD is approximately 40 weeks (280 days) from your LMP. This date is an estimate, and only about 5% of babies are born exactly on their due date. Most babies arrive between 37 and 42 weeks of gestation.
Current Pregnancy Week: By comparing your LMP to today's date, the calculator determines how many weeks and days you have been pregnant.
The Trimesters of Pregnancy
Pregnancy is divided into three trimesters, each with its unique developments and challenges:
First Trimester (Weeks 1-13): This period begins with conception and ends around week 13. It's a time of rapid development for the baby, with major organs forming. For the mother, symptoms like morning sickness, fatigue, and breast tenderness are common.
Second Trimester (Weeks 14-27): Often considered the "golden trimester," many women experience a decrease in early pregnancy symptoms and an increase in energy. The baby continues to grow and develop, and you might start to feel fetal movements (quickening).
Third Trimester (Weeks 28-40+): The final stretch! The baby grows significantly in size and weight, preparing for birth. The mother may experience increased discomfort, including back pain, swelling, and frequent urination.
Why Track Your Pregnancy Week by Week?
Tracking your pregnancy week by week offers several benefits:
Anticipate Milestones: Understand what developmental changes your baby is undergoing and what physical changes you might experience.
Prepare for Appointments: Knowing your current week helps you and your healthcare provider plan for necessary screenings, tests, and check-ups.
Emotional Preparedness: Staying informed can help reduce anxiety and empower you to make informed decisions throughout your pregnancy.
Planning and Preparation: From setting up the nursery to attending childbirth classes, a clear timeline helps with practical preparations.
Important Note: This calculator provides an estimate. Always consult with your healthcare provider for personalized medical advice, accurate dating, and comprehensive prenatal care.
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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.";
return;
}
var lmp = new Date(lmpDateInput);
// Set time to noon to avoid timezone issues with date calculations
lmp.setHours(12, 0, 0, 0);
var today = new Date();
today.setHours(12, 0, 0, 0);
// Check if LMP is in the future or too far in the past
if (lmp > today) {
resultDiv.innerHTML = "The LMP date cannot be in the future. Please enter a valid date.";
return;
}
var timeDiff = today.getTime() – lmp.getTime();
var daysPregnant = Math.floor(timeDiff / (1000 * 60 * 60 * 24));
if (daysPregnant today check, but good for robustness
resultDiv.innerHTML = "Invalid date range. Please ensure LMP is before today's date.";
return;
}
var currentWeek = Math.floor(daysPregnant / 7);
var currentDay = daysPregnant % 7;
// Estimated Due Date (EDD) is 280 days from LMP
var eddMillis = lmp.getTime() + (280 * 24 * 60 * 60 * 1000);
var edd = new Date(eddMillis);
var timeToEdd = edd.getTime() – today.getTime();
var daysRemaining = Math.ceil(timeToEdd / (1000 * 60 * 60 * 24));
var trimester = "";
if (currentWeek >= 0 && currentWeek = 14 && currentWeek = 28 && currentWeek 40) {
trimester = "Post-term (beyond 40 weeks)";
} else {
trimester = "Pre-conception or very early (less than 1 week)";
}
var eddFormatted = edd.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
var resultHtml = "
Your Pregnancy Details:
";
if (currentWeek < 1) {
resultHtml += "Based on your LMP, you are in the very early stages of pregnancy (less than 1 week pregnant) or potentially pre-conception.";
} else if (currentWeek > 42) {
resultHtml += "Based on your LMP, you are " + currentWeek + " weeks and " + currentDay + " days pregnant. This is beyond the typical 40-week gestation period. Please consult your healthcare provider.";
} else {
resultHtml += "You are currently " + currentWeek + " weeks and " + currentDay + " days pregnant.";
}
resultHtml += "This places you in your " + trimester + ".";
resultHtml += "Your Estimated Due Date (EDD) is: " + eddFormatted + ".";
if (daysRemaining > 0) {
resultHtml += "Approximately " + daysRemaining + " days remaining until your estimated due date.";
} else if (daysRemaining = 37) {
resultHtml += "Your estimated due date has passed or is very near. Your baby could arrive any day now!";
} else if (daysRemaining <= 0 && currentWeek < 37) {
resultHtml += "There might be an issue with the LMP date or the calculation. Please verify your LMP date.";
}
resultDiv.innerHTML = resultHtml;
}
// Set default LMP date to 10 weeks ago for a realistic example
window.onload = function() {
var today = new Date();
var defaultLMP = new Date(today.getTime() – (10 * 7 * 24 * 60 * 60 * 1000)); // 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;
calculatePregnancy(); // Run calculation on load with default value
};