How Many Weeks is Pregnancy Calculator

Pregnancy Weeks & Due Date Calculator

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Due Date (EDD)

Days to Go

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How Pregnancy Weeks are Calculated

Most healthcare providers calculate pregnancy starting from the first day of your last menstrual period (LMP). This is known as the gestational age. Even though you aren't actually pregnant for the first two weeks (ovulation usually occurs 14 days after your period starts), this standardized method provides a consistent starting point for tracking fetal development.

Understanding the Trimesters

A typical pregnancy lasts approximately 40 weeks (280 days) and is divided into three distinct phases:

• First Trimester: Weeks 1 through 13. This is a period of rapid development where all major organs begin to form.
• Second Trimester: Weeks 14 through 26. Often called the "golden phase," morning sickness usually fades, and you may begin to feel the baby move.
• Third Trimester: Weeks 27 through 40. The baby grows significantly in size and weight, preparing for life outside the womb.

Naegele's Rule for Due Dates

Our calculator uses Naegele's Rule, the standard mathematical formula for estimating a due date. It adds 280 days (40 weeks) to the first day of your last period. While only about 4% of babies are born on their exact due date, it serves as a helpful milestone for medical care.

Pregnancy Milestone Table

Week	Developmental Milestone
Week 5	The heart begins to beat.
Week 10	The embryo becomes a fetus.
Week 20	The halfway point; gender is often visible on ultrasound.
Week 24	Age of viability; lungs begin to produce surfactant.
Week 37	The pregnancy is considered "early term."

function calculatePregnancy() { var lmpInput = document.getElementById('lmpDate').value; var resultDiv = document.getElementById('calculator-results'); if (!lmpInput) { alert('Please select the first day of your last period.'); return; } var lmp = new Date(lmpInput); var today = new Date(); // Set times to midnight for accurate day calculation lmp.setHours(0,0,0,0); today.setHours(0,0,0,0); // Calculate total days passed var diffTime = today.getTime() – lmp.getTime(); var diffDays = Math.floor(diffTime / (1000 * 60 * 60 * 24)); if (diffDays 300) { alert('The date selected indicates a pregnancy older than 43 weeks. Please check the date.'); return; } // Calculations var weeks = Math.floor(diffDays / 7); var days = diffDays % 7; // Estimated Due Date (280 days from LMP) var edd = new Date(lmp.getTime()); edd.setDate(edd.getDate() + 280); var options = { month: 'long', day: 'numeric', year: 'numeric' }; var eddString = edd.toLocaleDateString(undefined, options); // Trimester logic var trimester = ""; if (weeks < 13) { trimester = "First Trimester"; } else if (weeks < 27) { trimester = "Second Trimester"; } else { trimester = "Third Trimester"; } // Days remaining var msPerDay = 1000 * 60 * 60 * 24; var daysToGo = Math.floor((edd.getTime() – today.getTime()) / msPerDay); if (daysToGo < 0) daysToGo = 0; // Percentage var percent = Math.min(((diffDays / 280) * 100).toFixed(1), 100); // Display Results document.getElementById('currentStage').innerHTML = weeks + " Weeks, " + days + " Days"; document.getElementById('trimesterResult').innerHTML = trimester; document.getElementById('dueDateResult').innerHTML = eddString; document.getElementById('daysToGo').innerHTML = daysToGo + " days"; document.getElementById('percentageDone').innerHTML = percent + "%"; resultDiv.style.display = 'block'; resultDiv.scrollIntoView({ behavior: 'smooth' }); }