Enter the first day of your last menstrual period (LMP) to estimate your due date. This uses Naegele’s rule: adds 280 days (40 weeks) to the LMP date.

Pregnancy Due Date Calculator: How to Estimate Your Baby’s Arrival

Expecting a baby has many exciting milestones, and one of the first is estimating your due date. A pregnancy due date calculator estimates when your little one might arrive. While it’s not an exact prediction—only about 4-5% of babies are born on their exact due date—it helps you plan prenatal care, maternity leave, and preparations.

This article explains how due dates are calculated, their accuracy, and factors that influence them. We also provide a simple, reliable pregnancy due date calculator tool you can use right now.

How Is a Pregnancy Due Date Calculated?

Healthcare providers use several methods to estimate your due date, known as the estimated date of delivery (EDD). The most common is Naegele’s rule, a standard method developed in the 19th century by German obstetrician Franz Naegele.

Naegele’s Rule Explained

Naegele’s rule assumes:

• A regular 28-day menstrual cycle.
• Ovulation and conception around day 14.
• Pregnancy lasting 280 days (40 weeks) from the first day of your last menstrual period (LMP).

To calculate:

1. Take the first day of your LMP.
2. Add 1 year.
3. Subtract 3 months.
4. Add 7 days.

Alternatively: Add 280 days directly to your LMP date.

Example: If your LMP was January 1, 2025:

• Add 1 year → January 1, 2026
• Subtract 3 months → October 1, 2025
• Add 7 days → October 8, 2025

This matches guidelines from reputable sources like the American College of Obstetricians and Gynecologists (ACOG), Mayo Clinic, and NHS.

Other Calculation Methods

• Conception date — Add 266 days (38 weeks) if you know exactly when conception occurred (common with IVF or ovulation tracking).
• Ultrasound dating — Early ultrasounds (especially in the first trimester) measure fetal size and often provide the most accurate estimate, sometimes adjusting the LMP-based date.
• IVF transfer — Specific adjustments based on embryo transfer day.

According to ACOG, first-trimester ultrasound is the gold standard for confirming gestational age when it differs significantly from LMP dating.

How Accurate Are Due Date Calculators?

Due dates are estimates. Studies show:

• Fewer than 5% of babies arrive exactly on their due date.
• About 60% are born within a week before or after.
• Nearly 90% within two weeks.

Full-term pregnancy ranges from 37 to 42 weeks. Factors affecting accuracy include:

• Irregular menstrual cycles.
• Variations in ovulation timing.
• Maternal age, ethnicity, parity (number of previous births), and weight.
• Modern research suggests Naegele’s rule may underestimate by a few days for some women (e.g., first-time moms averaging 288 days from LMP).

Ultrasound in early pregnancy improves accuracy, but even then, it’s not perfect. Always consult your healthcare provider—they may adjust your due date based on scans.

Important: A due date calculator is a helpful tool but not a substitute for professional medical advice. Your doctor or midwife provides the official estimate.

When to See Your Healthcare Provider

If your cycles are irregular or you’re unsure about your LMP, schedule an early ultrasound. Organizations like ACOG recommend confirming dating early for better prenatal care planning.

Pregnancy is unique—focus on a healthy lifestyle, regular check-ups, and enjoying the journey. Congratulations on your pregnancy!