Due Date Calculator: How to Calculate Your Baby's EDD (Naegele's Rule 2026)

Two lines on the test. A quick gasp, maybe a happy cry, maybe a stunned silence. And then, almost immediately, the question starts rolling in from family, friends, and coworkers: "So, when are you due?"

If you have no idea how to answer that yet, you are not alone. Most people learn how due dates are calculated only after they need one. The good news is the math is simple, and you can estimate your baby's arrival window with just one piece of information: the first day of your last menstrual period (LMP).

Here is how to calculate it precisely, what the number really means, and when to expect the key milestones along the way.

What Is EDD (Estimated Due Date)?

Your EDD, short for Estimated Due Date, is day 280 after the first day of your last period. That is 40 weeks, and it is the traditional benchmark obstetricians use to time appointments, tests, and ultrasound scans.

Here is the reality check most first-time parents do not hear early enough: only about 5 percent of babies are actually born on their EDD. The vast majority arrive within two weeks before or after that date, anywhere from 38 to 42 weeks. Think of the EDD less as a deadline and more as the center of a four-week landing zone.

Naegele's Rule Formula

The classic method is called Naegele's Rule, named after the 19th-century German obstetrician who popularized it. In its simplest form:

EDD = LMP + 280 days

Doctors often use a mnemonic version that is easier to do in your head:

EDD = LMP + 1 year - 3 months + 7 days

Example: If your LMP was January 15, 2026, add one year (January 15, 2027), subtract three months (October 15, 2026), then add seven days. Your EDD is October 22, 2026.

Both methods give the same answer. Use whichever is easier for you, or let a calculator do it in one click.

Why 280 Days?

The number is not arbitrary. Average human gestation is 38 to 42 weeks from conception. But you almost never know the exact date of conception. You do usually remember when your last period started, so obstetric dating uses that anchor instead.

LMP-based calculation works like this:

• About 2 weeks pass between LMP and ovulation (assuming a 28-day cycle)
• Then 38 weeks of actual gestation from conception to birth
• Total: 40 weeks, or 280 days from LMP

So when your doctor says you are "8 weeks pregnant," you have technically been carrying a developing embryo for about 6 weeks. The extra 2 weeks are counted for convenience, so everyone uses the same reference point.

Cycle Length Adjustment

Naegele's Rule assumes you have a textbook 28-day cycle. Many people do not, and that changes ovulation timing, which changes the true EDD.

The adjusted formula:

Adjusted EDD = LMP + 280 + (cycle length - 28)

Examples:

• 21-day cycle: subtract 7 days from EDD (you ovulate earlier)
• 35-day cycle: add 7 days to EDD (you ovulate later)
• 30-day cycle: add 2 days to EDD

If you have been tracking your cycle for a few months, use your average length. If your cycles vary wildly, an ultrasound will give a better estimate than any formula.

Pregnancy Weeks and Trimesters

Pregnancy is divided into three trimesters, each with its own set of changes and milestones:

Most people describe the second trimester as the "honeymoon" phase because nausea usually eases and energy returns. The third trimester brings physical discomfort back, but also the excitement of nesting and final prep.

Full Term Definition

Not all "on time" births are the same. In 2013, ACOG (the American College of Obstetricians and Gynecologists) refined the term categories to help doctors and parents understand risk profiles:

• Early term: 37 weeks to 38 weeks 6 days
• Full term: 39 weeks to 40 weeks 6 days (the ideal target range)
• Late term: 41 weeks to 41 weeks 6 days
• Post-term: 42 weeks and beyond

Babies born in the full-term window have the lowest rates of breathing problems, feeding issues, and NICU admission. Doctors generally avoid elective induction or C-section before 39 weeks unless medically necessary.

Key Pregnancy Milestones by Week

Here is a rough map of what happens when:

• Week 6 to 7: Heartbeat detectable via transvaginal ultrasound
• Week 12: End of first trimester, miscarriage risk drops significantly
• Week 20: Anatomy scan, you can usually find out your baby's sex
• Week 24: Viability threshold, survival possible with NICU support
• Week 28: Start of third trimester, glucose screening around now
• Week 32: Baby usually turns head-down for birth
• Week 37: Full-term begins, hospital bag should be packed

Every pregnancy is different, so treat these as general guides. Your OB will personalize the schedule based on your health history and how the pregnancy is progressing.

When Naegele's Rule Isn't Accurate

The formula assumes a lot: regular ovulation, a 28-day cycle, and a clear memory of your last period. Real life is messier. Naegele's Rule may be off by a week or more if you have:

• Irregular cycles (PCOS, thyroid conditions, high stress)
• Uncertain or unknown LMP date
• Recently stopped hormonal birth control (pill, IUD, implant)
• Very long or very short cycles (below 21 days or above 45 days)
• Conceived while breastfeeding

Solution: a first-trimester ultrasound between weeks 8 and 13 gives the most accurate dating available. At that stage, all embryos grow at nearly the same rate, so the crown-rump length measurement is highly reliable, often within 3 to 5 days. If the ultrasound date differs from your LMP date by more than a week, your doctor will usually go with the ultrasound.

Preparing for Baby by Trimester

Knowing your due date lets you plan backward. Here is a simple checklist:

First trimester (weeks 1 to 12):

• Start prenatal vitamins with at least 400 mcg folate
• Schedule your first OB or midwife appointment
• Cut alcohol, smoking, and limit caffeine to 200 mg per day
• Rest more than you think you need

Second trimester (weeks 13 to 27):

• Anatomy scan around week 20
• Glucose tolerance test around week 24 to 28
• Start your baby registry and think about childbirth classes
• Announce publicly if you are comfortable

Third trimester (weeks 28 to 40):

• Pack your hospital bag by week 36
• Write a birth plan and discuss it with your provider
• Install the infant car seat (many hospitals will not discharge you without one)
• Choose a pediatrician and schedule a "meet and greet"

Try the gunainaja Due Date Calculator

Instead of doing the math on a napkin, let a tool handle it. The gunainaja Due Date Calculator takes your LMP and cycle length, then instantly returns your EDD, current weeks pregnant, trimester, and a highlighted timeline of upcoming milestones.

It is free, works on mobile, and does not store any of your personal data. Bookmark /en/tools/kalkulator-hpl and share it with a friend who just saw two lines.

Related Tool: Healthy Pregnancy Weight

Pregnancy weight gain matters, both for your comfort and your baby's health. ACOG recommends 25 to 35 pounds (about 11 to 16 kg) of total gain for people who started pregnancy at a normal BMI. Underweight mothers should aim higher, and those with a higher starting BMI should aim lower.

Not sure where you started? Check with the BMI Calculator to find your pre-pregnancy category, then talk to your provider about a personalized weight gain target.

Closing

A due date is a beginning, not a deadline. It is a rough anchor that helps you plan appointments, prep the nursery, and mentally prepare for the biggest life change most people ever go through. Whether your baby shows up two weeks early or one week late, they are still right on time for their own story.

Take a breath. You have got this.

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Medical disclaimer: This calculator provides estimates. Always consult your OB/GYN or midwife for personalized pregnancy care.