How to Calculate Your Expected Due Date

A standard human pregnancy lasts approximately 40 weeks (280 days) from the first day of your last menstrual period (LMP). Follow these steps to find your estimated due date (EDD):

1. Select the first day of your last period (LMP) — This date is used as the official baseline for gestational age.
2. Adjust cycle length — The standard menstrual cycle is 28 days. If your cycle is shorter or longer, adjust the value (e.g., 30 days) to refine the estimate.
3. Click “Calculate Due Date” — The tool will display your estimated due date, gestational age (how far along you are), and remaining days.

The calculator also builds a pregnancy timeline marking key milestones, including estimated conception and trimester transition points.

Naegele’s Rule: The Mathematical Formula

Our calculator implements Naegele’s Rule, which is the standard clinical method to estimate due dates:

1. Take the date of the first day of your last menstrual period.
2. Add one year.
3. Subtract three months.
4. Add seven days.
5. Adjust for cycle length (add or subtract the difference of your cycle from 28 days).

For example, if your LMP was October 15, 2025 and you have a 28-day cycle:

• October 15, 2025 + 1 year = October 15, 2026.
• Subtract 3 months = July 15, 2026.
• Add 7 days = July 22, 2026.

Understanding Gestational Age vs. Conception Age

Doctors track pregnancy based on **gestational age** (weeks since your last period). This means that during the first two weeks of your “pregnancy,” you are not actually pregnant yet — your body is preparing for ovulation. **Conception age** measures the time since the egg was actually fertilized (usually about 2 weeks after your LMP). Gestational age is the medical standard because it is difficult to determine the exact date of conception.

Frequently Asked Questions

What percentage of babies are born on their actual due date?

Only about 4% to 5% of babies are born exactly on their estimated due date. Most babies arrive within 10 days before or after the target date. A pregnancy is considered full-term between 37 weeks and 42 weeks.

How does cycle length affect my due date?

If your menstrual cycle is longer than the standard 28 days, ovulation occurs later, which pushes your due date back. If your cycle is shorter, ovulation occurs earlier, moving your due date forward. Our calculator accounts for this by adjusting Naegele’s baseline accordingly.

What is a trimester?

Pregnancy is divided into three trimesters, each lasting about 12 to 14 weeks:

• First Trimester (Weeks 1-12): A period of rapid development. The baby’s heart, brain, and limbs begin to form.
• Second Trimester (Weeks 13-27): Often called the “golden trimester” as morning sickness typically fades. You will begin to feel the baby move.
• Third Trimester (Weeks 28-40+): The baby continues to grow and prepare for birth.

Can my due date change later in pregnancy?

Yes. Early ultrasound scans (usually performed in the first trimester) measure the baby’s crown-rump length to provide a highly accurate estimation of gestational age. If the ultrasound estimate differs from your LMP date by more than a few days, your doctor may adjust your official due date.