Pregnancy Due Date Calculator Guide: How Your EDD Is Calculated

What Is an Estimated Due Date (EDD)?

Your estimated due date is the projected date your baby is expected to be born. “Estimated” is the key word. Normal, healthy pregnancies end anywhere from 37 to 42 weeks. The EDD is a statistical midpoint, not a hard deadline.

According to the American College of Obstetricians and Gynecologists (ACOG), gestational age is counted from the first day of your last menstrual period — not from conception. By the time an egg is fertilized, you're already considered about 2 weeks pregnant in clinical terms.

Naegele's Rule: The Standard Calculation

The most common method for calculating an EDD is Naegele's Rule, named after 19th-century German obstetrician Franz Karl Naegele. The formula:

EDD = LMP + 280 days (40 weeks)

There's a mental shortcut too: add 7 days to the first day of your LMP, then add 9 months (or subtract 3 months and add 7 days). Both give the same result.

Example: LMP = January 1, 2026. Add 7 days → January 8. Add 9 months → October 8, 2026.

Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycles are shorter, longer, or irregular, this estimate can be off by days or even weeks. A 2022 study in the American Journal of Obstetrics and Gynecology found LMP-based dating misclassified gestational age in roughly 20% of pregnancies compared to first-trimester ultrasound.

Gestational Age vs. Fetal Age: What's the Difference?

These two terms confuse a lot of people, and the distinction matters clinically.

• Gestational age counts from the first day of your LMP. This is the number your doctor uses. At the moment of conception (roughly day 14 of a 28-day cycle), gestational age is already 2 weeks.
• Fetal age (also called embryonic age or fertilization age) counts from the actual date of conception. It runs approximately 2 weeks behind gestational age.

So when your doctor says “you're 10 weeks pregnant,” they mean 10 weeks gestational age — your embryo is actually about 8 weeks old in fetal age terms. Always clarify which measurement is being used when discussing your pregnancy timeline.

4 Methods to Calculate Your Due Date

1. Last Menstrual Period (LMP) — Naegele's Rule

The simplest and most widely used method. Requires you to know the first day of your last period. Works best with regular 28-day cycles.

2. First-Trimester Ultrasound (Crown-Rump Length)

ACOG considers first-trimester ultrasound performed before 13 weeks 6 days the gold standard for pregnancy dating. The sonographer measures the embryo's crown-rump length (CRL) — from the top of the head to the bottom — and cross-references it with established growth charts. Accuracy: ±5–7 daysat 8–12 weeks.

If the ultrasound EDD differs from LMP by more than 7 days in the first trimester, ACOG recommends updating the official EDD to the ultrasound date.

3. Conception Date

If you tracked ovulation precisely (basal body temperature, OPK strips, or fertility monitoring), you can add 266 days (38 weeks) to the conception date. The NICHD notes the fertilization-to-birth interval averages 268 days, with a standard deviation of about 10 days.

4. IVF Transfer Date

For IVF, the fertilization date is known exactly, making EDD calculation highly reliable:

• 5-day blastocyst transfer: Transfer date + 261 days = EDD
• 3-day embryo transfer: Transfer date + 263 days = EDD

According to the CDC Assisted Reproductive Technology Surveillance Report (2021), IVF accounts for roughly 2.3% of all U.S. births — over 85,000 babies per year.

Trimester Breakdown: Weeks, Milestones, and What to Expect

5 Things to Expect in Each Trimester

First trimester (weeks 1–12):

• Morning sickness that can strike at any time of day (affects ~70% of pregnant people)
• Extreme fatigue as your body ramps up progesterone production
• Your first prenatal appointment, typically at 8–10 weeks
• First ultrasound to confirm heartbeat and establish EDD
• Genetic screening options (NIPT, nuchal translucency scan) discussed around weeks 10–13

Second trimester (weeks 13–26):

• Nausea usually subsides — energy returns for most people
• Anatomy scan (anomaly scan) at 18–22 weeks checks all major structures
• Fetal movement (“quickening”) typically felt between weeks 16–22
• Glucose challenge test for gestational diabetes screening around weeks 24–28
• Visible baby bump; weight gain accelerates (recommended: about 1 lb/week)

Third trimester (weeks 27–40+):

• Baby triples in weight from week 28 to birth
• Fetal kick counts become important — 10 movements in 2 hours is typical
• Group B Streptococcus (GBS) swab test at 35–37 weeks
• Cervical checks begin; baby “drops” (engages) in the pelvis
• Birth plan discussion with your provider; hospital registration

Full-Term Definitions: What ACOG Says

In 2013, ACOG redefined the terminology around gestational age at delivery to improve outcomes. The previous catch-all term “term” (37–42 weeks) was replaced with four distinct categories:

This reclassification matters because research showed that early-term babies (37–38 weeks) have meaningfully higher rates of respiratory distress, NICU admission, and feeding difficulties than those born at 39–40 weeks. The EDD isn't just a calendar curiosity — it directly influences clinical decision-making.

How Accurate Is an EDD? The Statistics

The due date is a probability estimate, not a prediction. Here is how births actually distribute:

ACOG notes that only about 5% of babies are born on their exact estimated due date, and approximately 80% are born within 2 weeks of EDD. A landmark 2013 NICHD-funded study in Human Reproduction found that the natural variation in pregnancy length spans up to 37 days, even after controlling for ovulation timing.

The CDC's National Center for Health Statistics reported that in 2022, 10.4% of U.S. births were preterm(before 37 completed weeks) — the highest rate in over a decade. Preterm birth is the leading cause of infant death in the United States.

Ultrasound Dating Accuracy by Trimester

The takeaway: get your dating ultrasound early. A scan at 8–10 weeks is almost twice as accurate as one at 20 weeks for establishing gestational age. If you miss the first-trimester window, a 20-week anatomy scan can still provide useful data but with wider error bars.

Key Statistics at a Glance

• Average human pregnancy: 280 days from LMP, or ~266 days from conception (NICHD, 2013)
• Only 5% of babies are born on their exact EDD (ACOG)
• About 80% of births occur within 2 weeks of the EDD
• First-trimester ultrasound accuracy: ±5–7 days (ACOG Practice Bulletin, 2017)
• Preterm birth rate in the U.S.: 10.4% (CDC National Center for Health Statistics, 2022)
• IVF accounts for roughly 2.3% of all U.S. births per year (CDC ART Surveillance, 2021)

Related Guides

• Pregnancy Calculator: Tracking Gestational Age Week by Week
• Ovulation Calculator: How to Identify Your Fertile Window
• Pregnancy Weight Gain: How Much Is Normal by Trimester
• Fertility Window: When Are You Most Likely to Conceive?

Frequently Asked Questions

How is a due date calculated?

The standard method is Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). A quick shortcut: add 7 days to your LMP, then add 9 months (or subtract 3 months and add 7 days). This assumes a 28-day cycle with ovulation on day 14. If your cycles are irregular, first-trimester ultrasound dating is more accurate than LMP alone.

How accurate is a due date calculator?

An LMP-based calculator is a solid starting point, but only about 5% of babies are born on their exact EDD (ACOG), and approximately 80% are born within 2 weeks. First-trimester ultrasound is the most accurate method, with a margin of error of only ±5 to 7 days (ACOG Practice Bulletin, 2017). The EDD is best treated as the midpoint of a 2–3 week delivery window, not a precise prediction.

What is considered full term in pregnancy?

ACOG's 2013 guidelines define full termas 39 weeks 0 days through 40 weeks 6 days. Early term is 37 weeks 0 days through 38 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term is 42 weeks and beyond. Babies born early term have higher rates of respiratory distress and NICU admission than those born at full term — so the distinction matters clinically.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of your last menstrual period (LMP) — the standard clinical measure. Fetal age (embryonic age) counts from actual conception, roughly 2 weeks after LMP in a typical cycle. When your doctor says you're “10 weeks pregnant,” that's 10 weeks gestational age; the embryo itself is approximately 8 weeks old.

When should I go to the hospital if I think I'm in labor?

Most providers recommend the 5-1-1 rulefor first-time mothers: contractions every 5 minutes, lasting 1 minute each, for at least 1 hour. For subsequent pregnancies, the window is often 7-1-1 or sooner. Go immediately if your water breaks, you have heavy bleeding, severe headache, vision changes, or notice reduced fetal movement. Always follow your OB-GYN's or midwife's specific guidance — they know your individual situation.