Pregnancy Week Calculator by Lmp

Calculate your due date and gestational age based on your Last Menstrual Period (LMP)

Estimated Due Date –

Gestational Age –

Current Trimester –

Understanding Your Pregnancy Week Calculation

Pregnancy typically lasts about 40 weeks (280 days) from the first day of your last menstrual period (LMP). Most healthcare providers use the LMP method to determine your due date, even though conception usually occurs about two weeks after your period starts.

How the LMP Method Works

The standard formula used by this calculator is based on Naegele's Rule. This assumes a standard 28-day menstrual cycle. If your cycle is consistently longer or shorter than 28 days, our calculator adjusts the estimated due date (EDD) to provide a more accurate window for your delivery.

Pregnancy Milestones by Trimester

• First Trimester: Weeks 1 through 12. This is a period of rapid development where major organs form.
• Second Trimester: Weeks 13 through 26. Often called the "golden period," many people feel more energetic during these months.
• Third Trimester: Week 27 through birth. The baby grows significantly in weight and prepares for life outside the womb.

Frequently Asked Questions

What if I don't know my LMP?
If your periods are irregular or you can't remember the exact date, your doctor will likely perform a dating ultrasound (usually between 8 and 14 weeks) to measure the fetus and establish a more accurate due date.

How accurate is this calculator?
While this calculator provides a scientifically sound estimate based on your inputs, only about 5% of babies are born exactly on their due date. Most births occur between 37 and 42 weeks.

function calculatePregnancy() { var lmpInput = document.getElementById("lmpDate").value; var cycleLength = parseInt(document.getElementById("cycleLength").value); if (!lmpInput) { alert("Please select the date of your last menstrual period."); return; } var lmp = new Date(lmpInput); var today = new Date(); // Adjust for cycle length (Standard is 28) var cycleAdjustment = cycleLength – 28; // Calculate Due Date: LMP + 280 days + cycle adjustment var edd = new Date(lmp.getTime()); edd.setDate(edd.getDate() + 280 + cycleAdjustment); // Calculate Gestational Age var diffTime = Math.abs(today – lmp); var diffDays = Math.ceil(diffTime / (1000 * 60 * 60 * 24)); var weeks = Math.floor(diffDays / 7); var days = diffDays % 7; // Display Area document.getElementById("resultsArea").style.display = "block"; // Format Due Date var options = { year: 'numeric', month: 'long', day: 'numeric' }; document.getElementById("eddResult").innerText = edd.toLocaleDateString(undefined, options); // Format Gestational Age if (weeks < 0) { document.getElementById("ageResult").innerText = "Not yet pregnant"; } else { document.getElementById("ageResult").innerText = weeks + " Weeks, " + days + " Days"; } // Determine Trimester var trimester = ""; if (weeks < 13) { trimester = "First"; } else if (weeks = 40) { progressText = "You have reached your full-term due date! Your baby could arrive any time now."; } else if (weeks < 0) { progressText = "The date entered is in the future. Please select the first day of your last actual period."; } else { var totalDays = 280; var currentTotalDays = (weeks * 7) + days; var percent = Math.min(Math.round((currentTotalDays / totalDays) * 100), 100); progressText = "You are approximately " + percent + "% of the way through your pregnancy journey!"; } document.getElementById("progressSummary").innerText = progressText; // Smooth scroll to results document.getElementById("resultsArea").scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }