Trimester Calculator

Pregnancy Trimester Calculator

Enter either your Last Menstrual Period (LMP) start date or your Estimated Due Date (EDD) to calculate your current trimester and a full pregnancy breakdown.

function calculateTrimesters() { var MS_PER_DAY = 1000 * 60 * 60 * 24; var MS_PER_WEEK = MS_PER_DAY * 7; var lmpDateStr = document.getElementById("lmpDate").value; var eddDateStr = document.getElementById("eddDate").value; var lmpDate, eddDate; if (lmpDateStr) { lmpDate = new Date(lmpDateStr + "T00:00:00"); // Ensure consistent timezone interpretation eddDate = new Date(lmpDate.getTime() + (280 * MS_PER_DAY)); } else if (eddDateStr) { eddDate = new Date(eddDateStr + "T00:00:00"); lmpDate = new Date(eddDate.getTime() – (280 * MS_PER_DAY)); } else { document.getElementById("result").innerHTML = "Please enter either your Last Menstrual Period start date or your Estimated Due Date."; return; } if (isNaN(lmpDate.getTime()) || isNaN(eddDate.getTime())) { document.getElementById("result").innerHTML = "Invalid date entered. Please use YYYY-MM-DD format."; return; } var today = new Date(); today.setHours(0, 0, 0, 0); // Normalize today's date to start of day var pregnancyDurationMs = today.getTime() – lmpDate.getTime(); var currentWeek = Math.floor(pregnancyDurationMs / MS_PER_WEEK) + 1; // +1 because week 0 is week 1 var trimester1Start = lmpDate; var trimester1End = new Date(lmpDate.getTime() + (13 * MS_PER_WEEK) – MS_PER_DAY); // End of week 13 var trimester2Start = new Date(lmpDate.getTime() + (13 * MS_PER_WEEK)); // Start of week 14 var trimester2End = new Date(lmpDate.getTime() + (27 * MS_PER_WEEK) – MS_PER_DAY); // End of week 27 var trimester3Start = new Date(lmpDate.getTime() + (27 * MS_PER_WEEK)); // Start of week 28 var trimester3End = eddDate; // End of week 40 (due date) var currentTrimester = ""; if (currentWeek >= 1 && currentWeek = 14 && currentWeek = 28 && currentWeek 40) { currentTrimester = "Past Due Date (Week " + currentWeek + ")"; } else { currentTrimester = "Not yet pregnant or date is in the future."; } var resultHtml = "

Your Pregnancy Overview:

"; resultHtml += "Last Menstrual Period (LMP) Start Date: " + formatDate(lmpDate) + ""; resultHtml += "Estimated Due Date (EDD): " + formatDate(eddDate) + ""; resultHtml += "Current Week of Pregnancy: " + currentWeek + " weeks"; resultHtml += "Current Trimester: " + currentTrimester + ""; resultHtml += "

Trimester Breakdown:

"; resultHtml += "First Trimester (Weeks 1-13): " + formatDate(trimester1Start) + " to " + formatDate(trimester1End) + ""; resultHtml += "Second Trimester (Weeks 14-27): " + formatDate(trimester2Start) + " to " + formatDate(trimester2End) + ""; resultHtml += "Third Trimester (Weeks 28-40): " + formatDate(trimester3Start) + " to " + formatDate(trimester3End) + ""; document.getElementById("result").innerHTML = resultHtml; } function formatDate(date) { var options = { year: 'numeric', month: 'long', day: 'numeric' }; return date.toLocaleDateString('en-US', options); }

Understanding Pregnancy Trimesters

Pregnancy is a remarkable journey, typically lasting about 40 weeks from the first day of your last menstrual period (LMP). To help track development and manage expectations, this period is commonly divided into three trimesters, each roughly three months long. While these divisions are approximate, they offer a useful framework for understanding the significant changes happening to both the mother and the developing baby.

How Trimesters Are Calculated

The standard way to calculate trimesters is based on your LMP. Your estimated due date (EDD) is typically 280 days (40 weeks) from your LMP. The trimesters are then broken down as follows:

• First Trimester: From Week 1 (LMP) to the end of Week 13.
• Second Trimester: From Week 14 to the end of Week 27.
• Third Trimester: From Week 28 until birth (typically Week 40).

This calculator uses these standard definitions to provide you with an accurate breakdown of your pregnancy timeline.

The First Trimester (Weeks 1-13)

This trimester begins with the first day of your last menstrual period, even though conception usually occurs around week 2 or 3. It's a period of rapid development for the baby, with all major organs forming. For the mother, symptoms like morning sickness, fatigue, and breast tenderness are common. It's a critical time for establishing healthy habits and beginning prenatal care.

Example: If your LMP was January 1st, your first trimester would end around April 1st.

The Second Trimester (Weeks 14-27)

Often called the "golden trimester," many women find relief from early pregnancy symptoms during this time. The baby continues to grow significantly, and you might start to feel the first flutters of movement (quickening). This is also when many prenatal tests and ultrasounds occur, allowing you to see your baby's development. Your belly will likely become more noticeable.

Example: Continuing from the previous example, your second trimester would run from early April to early July.

The Third Trimester (Weeks 28-40)

The final stretch! Your baby is rapidly gaining weight and maturing its lungs and brain. You might experience increased fatigue, backaches, and Braxton Hicks contractions as your body prepares for labor. Regular prenatal visits become more frequent, focusing on monitoring your health and the baby's position. The anticipation of meeting your baby builds during these last weeks.

Example: Following the same timeline, your third trimester would begin in early July and continue until your estimated due date around October 8th.

Why Trimesters Matter

Knowing which trimester you are in helps healthcare providers tailor your prenatal care, screenings, and advice. It also helps expectant parents understand the developmental milestones of their baby and anticipate the physical and emotional changes they might experience. While these are general guidelines, every pregnancy is unique, and your healthcare provider is your best resource for personalized information and support.

Disclaimer: This calculator provides an estimated timeline based on standard medical calculations. It is not a substitute for professional medical advice. Always consult with your healthcare provider for accurate dating and personalized pregnancy care.