Pregnancy Due Date Calculator

How Due Date Estimation Works

This calculator uses Naegele's rule, the standard method doctors have relied on for over 200 years. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). The underlying assumption is a 28-day cycle with ovulation occurring on day 14. If you know your actual conception date, you can use that instead — the calculator will work backward to a theoretical LMP and forward to your due date. You can also enter a due date from your doctor to reverse-calculate your LMP and conception date, which is useful for tracking milestones.

Understanding the Trimester Timeline

Pregnancy is divided into three trimesters, each with distinct developmental milestones. The first trimester (weeks 1-12) is when major organs begin forming — by week 8, the baby's heartbeat is typically detectable on ultrasound. The second trimester (weeks 13-26) is often called the "golden period" — nausea usually subsides, and you may feel the baby's first movements around week 16-20. The anatomy scan at week 20 marks the halfway point. The third trimester (weeks 27-40) is when the baby gains most of its weight, practices breathing, and prepares for birth. Viability outside the womb is generally reached around week 24.

Keep in Mind

Only about 5% of babies arrive on their exact estimated due date. Most births occur within a two-week window before or after the calculated date. Your healthcare provider may adjust your due date based on early ultrasound measurements, especially if your menstrual cycles are irregular. This calculator is an educational tool — always consult your doctor or midwife for personalized medical advice and prenatal care.

Related Tools

Track days between events with the Date Difference Calculator. Monitor your health with the BMI Calculator and Calorie Calculator. Set reminders with the Countdown Timer.