Pregnancy Weeks Calculator

Pregnancy Weeks & Due Date Calculator

Your Results

Current Progress: –

Estimated Due Date: –

Current Trimester: –

Days Remaining: –

Please enter a valid date.

function calculatePregnancy() { var lmpInput = document.getElementById("lmpDate").value; var resultDiv = document.getElementById("pregnancyResults"); var errorDiv = document.getElementById("errorBox"); if (!lmpInput) { errorDiv.style.display = "block"; resultDiv.style.display = "none"; return; } var lmpDate = new Date(lmpInput); var today = new Date(); today.setHours(0,0,0,0); // Calculate Due Date (280 days from LMP) var dueDate = new Date(lmpDate.getTime()); dueDate.setDate(dueDate.getDate() + 280); // Calculate difference in time var timeDiff = today.getTime() – lmpDate.getTime(); var daysPassed = Math.floor(timeDiff / (1000 * 3600 * 24)); if (daysPassed 300) { errorDiv.textContent = "The date entered must be within the last 42 weeks."; errorDiv.style.display = "block"; resultDiv.style.display = "none"; return; } errorDiv.style.display = "none"; resultDiv.style.display = "block"; var weeks = Math.floor(daysPassed / 7); var days = daysPassed % 7; // Text results document.getElementById("currentProgress").innerText = weeks + " Weeks, " + days + " Days"; document.getElementById("dueDateDisplay").innerText = dueDate.toLocaleDateString('en-US', { month: 'long', day: 'numeric', year: 'numeric' }); // Trimester logic var trimester = ""; if (weeks < 13) { trimester = "First Trimester"; } else if (weeks 0 ? daysLeft : "0"; }

How the Pregnancy Weeks Calculator Works

A pregnancy weeks calculator is an essential tool for expecting parents to track fetal development and plan for the arrival of their baby. Most medical professionals use the Last Menstrual Period (LMP) method to estimate the progress of a pregnancy.

The Naegele's Rule

Standard pregnancy calculations are based on Naegele's rule, which assumes a standard 28-day menstrual cycle. Because the exact date of conception is often difficult to pinpoint, doctors count the beginning of pregnancy from the first day of your last period. This means that during the first two weeks of your "pregnancy," you are not actually pregnant yet—your body is preparing for ovulation.

Pregnancy Trimester Breakdown

• First Trimester (Weeks 1-12): This is the period of rapid development where major organs begin to form.
• Second Trimester (Weeks 13-26): Often called the "golden period," many women feel an increase in energy as the baby continues to grow.
• Third Trimester (Weeks 27-40): The final stretch where the baby gains weight and prepares for birth.

Common Calculation Example

If your last period started on January 1st:

• Add 280 days: Your estimated due date would be October 8th.
• Current Progress: If today is February 12th, you would be 6 weeks and 0 days pregnant.

Why is my doctor's date different?

While a pregnancy weeks calculator provides a very accurate estimate based on dates, your healthcare provider might adjust your due date after an early ultrasound. An ultrasound measures the "crown-rump length" of the fetus, which is often a more precise indicator of gestational age than the LMP, especially for those with irregular cycles.