Pregnancy Calculator: How to Estimate Your Due Date and Track Each Trimester

Finding out you’re expecting is one of life’s most exciting moments — and the first question most parents ask is “When is my baby due?” A pregnancy calculator answers that, tracks how far along you are week by week, and breaks the journey into trimesters. Here’s how due dates are actually calculated and how to use the tool correctly.

Use our free Pregnancy Calculator — enter your last menstrual period or conception date and get your due date, current week, and trimester instantly.

How a Pregnancy Calculator Works

Most pregnancy calculators are built around Naegele’s rule, the standard formula obstetricians use to predict due dates. It assumes a typical 28-day cycle with ovulation around day 14, and counts pregnancy from the first day of your last menstrual period (LMP) — not from the day you conceived. Under this system, your due date is about 280 days (40 weeks) after the first day of your LMP.

What You’ll Need to Enter

• The first day of your last menstrual period (LMP) — the most common input
• Your average cycle length — normally 28 days, but many women have longer or shorter cycles
• Some calculators also accept a conception date if you tracked ovulation

What the Results Tell You

• Estimated due date — the 40-week target, though only about 5% of babies arrive exactly on it
• Current week and day of pregnancy — e.g. “12 weeks, 3 days”
• Trimester breakdown — first (weeks 1-13), second (14-27), third (28-40)
• Remaining time — weeks until your due date

Adjusting for Cycle Length

Naegele’s rule assumes a 28-day cycle. If your cycles are longer or shorter, the estimated due date shifts accordingly: add one day for each day your average cycle exceeds 28 days, subtract for shorter cycles. For example, with a 32-day cycle, your due date moves roughly 4 days later than the standard calculation. Entering your actual cycle length in the Pregnancy Calculator gives a more personalized estimate.

Why the Due Date Isn’t Exact

The due date is an estimate, not a promise. Ovulation timing varies, sperm can survive several days, and conception itself doesn’t always happen the day you think. Your healthcare provider will typically confirm or adjust the date with an early ultrasound, which measures the embryo’s size — the most accurate dating method in the first trimester. A due date calculated later in pregnancy is less reliable because fetal growth rates vary.

Track Week by Week

Beyond the due date, knowing your current week helps you understand what to expect: when morning sickness typically fades (around week 13), when the second-trimester energy surge arrives, and when to schedule key screenings. Run the numbers in the Pregnancy Calculator whenever you need a quick reference — then discuss the details with your doctor or midwife at your prenatal visits.