How to Calculate Pregnancy

Pregnancy Due Date Calculator

Standard cycle is 28 days.

Results

Estimated Due Date –

Current Progress –

Trimester –

Days to Go –

*This is an estimate based on your menstrual cycle. Please consult your doctor for medical accuracy.

function calculatePregnancy() { var lmpInput = document.getElementById("lmpDate").value; var cycleLength = parseInt(document.getElementById("cycleLength").value); var resultDiv = document.getElementById("pregnancyResult"); if (!lmpInput) { alert("Please select the date of your last period."); return; } var lmpDate = new Date(lmpInput); var today = new Date(); // Naegele's Rule: LMP + 280 days // Adjustment: + (Cycle Length – 28) var cycleAdjustment = cycleLength – 28; var edd = new Date(lmpDate); edd.setDate(edd.getDate() + 280 + cycleAdjustment); // Gestational Age calculation var diffTime = Math.abs(today – lmpDate); var diffDays = Math.ceil(diffTime / (1000 * 60 * 60 * 24)); var weeks = Math.floor(diffDays / 7); var days = diffDays % 7; // Trimester logic var trimester = ""; if (weeks < 13) { trimester = "First Trimester"; } else if (weeks 0 ? countdownDays : 0) + " Days"; resultDiv.style.display = "block"; resultDiv.scrollIntoView({ behavior: 'smooth' }); }

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How to Calculate Your Pregnancy Due Date

Understanding your pregnancy timeline is essential for planning prenatal care and preparing for your new arrival. Most healthcare providers use the first day of your last menstrual period (LMP) as the starting point for calculating your pregnancy, even though you likely didn't conceive until about two weeks later.

The Science: Naegele's Rule

The standard method for calculating a pregnancy due date is Naegele's Rule. This formula assumes a standard human pregnancy lasts approximately 280 days (40 weeks) from the first day of the last menstrual period. The simple math is:

• Start with the first day of your LMP.
• Add 7 days.
• Subtract 3 months.
• Add 1 year.

Why Cycle Length Matters

Naegele's Rule assumes a perfect 28-day menstrual cycle where ovulation occurs exactly on day 14. However, many women have cycles that are longer or shorter. Our calculator allows for this adjustment. If you have a 32-day cycle, you likely ovulated 4 days later than the "average," meaning your due date will be roughly 4 days later than the standard calculation.

The Three Trimesters

Pregnancy is divided into three stages called trimesters, each characterized by specific developmental milestones:

Trimester	Timeframe	Key Development
First	Weeks 1 – 12	Organ formation and heartbeat starts.
Second	Weeks 13 – 26	Rapid growth; baby's movement is felt.
Third	Week 27 – Birth	Lung development and weight gain.

Frequently Asked Questions

Is the due date accurate?
Only about 4% to 5% of babies are born on their exact due date. Most babies arrive between 37 and 42 weeks of pregnancy. The due date is best viewed as a target window rather than a fixed appointment.

What if I don't know my LMP?
If your periods are irregular or you don't remember the date, your doctor will perform a dating ultrasound (usually in the first trimester). This measures the "crown-rump length" of the embryo, which is currently the most accurate way to determine gestational age in early pregnancy.

How many days is a full-term pregnancy?
A full-term pregnancy is generally considered to be 280 days, or 40 weeks. However, "early term" starts at 37 weeks, and "late term" begins at 41 weeks.