Due Date Calculator: How Pregnancy Due Dates Are Calculated

What Is an Estimated Due Date?

An estimated due date (EDD) is the calendar date on which your pregnancy is expected to reach 40 weeks. It is an estimate, not a deadline. Most pregnancies deliver somewhere in a 5-week window — two weeks before and two weeks after the EDD.

The due date gives your care team a reference point for tracking fetal development, scheduling screenings, and making decisions about induction or cesarean delivery if pregnancy continues past 41 or 42 weeks.

Naegele's Rule: The Standard Calculation Method

The most widely used method for calculating a due date is Naegele's Rule, developed in the 19th century by German obstetrician Franz Karl Naegele. Despite its age, it remains the clinical standard today.

The formula: LMP + 7 days − 3 months + 1 year = EDD

In simpler terms: add 280 days (40 weeks) to the first day of your last menstrual period. A full-term pregnancy is defined as 39 to 40 completed weeks of gestation.

Step-by-Step Example

• Last menstrual period (LMP): January 15, 2026
• Add 7 days: January 22, 2026
• Subtract 3 months: October 22, 2025
• Add 1 year: October 22, 2026

That's your EDD. The math is straightforward, but it assumes a 28-day menstrual cycle with ovulation on day 14. Women with irregular cycles may get a more accurate estimate from ultrasound.

According to the American College of Obstetricians and Gynecologists (ACOG), only 5% of women deliver on their exact EDD. The vast majority — roughly 80% — deliver within two weeks before or after that date.

How to Calculate Due Date from Conception

If you know your conception date (fertilization date), you can also calculate your due date directly. Conception typically occurs around day 14 of a 28-day cycle, approximately two weeks after the start of the LMP.

The formula: Conception date + 266 days = EDD

This is why LMP-based and conception-based due dates usually match: LMP + 280 days is the same as (LMP + 14 days) + 266 days, assuming average ovulation timing. If your ovulation is earlier or later than day 14, the conception-based method will be more accurate.

For most people, the LMP method is easier since conception date is rarely known with precision. Our Due Date Calculator supports both LMP and conception date inputs.

Ultrasound Dating vs LMP: Which Is More Accurate?

Ultrasound dating — particularly in the first trimester — is more accurate than LMP calculation for most women. Here's why: the LMP method assumes a perfect 28-day cycle, but cycles vary between 21 and 35 days. A woman with a 35-day cycle ovulates around day 21, not day 14, which would shift her EDD by a full week.

Crown-Rump Length (CRL) Measurement

Between 8 and 13 weeks of pregnancy, ultrasound technicians measure the crown-rump length (CRL) of the embryo. This single measurement is accurate to within 5 to 7 daysof the true gestational age. It's the gold standard for first-trimester dating.

After 14 weeks, accuracy decreases. A second-trimester ultrasound is typically accurate to within 10 to 14 days. Third-trimester ultrasounds are accurate only to within 3 weeks, which is why ACOG recommends against revising the EDD based on late ultrasound findings alone.

If first-trimester ultrasound and LMP disagree by more than 7 days, most providers will change the official EDD to the ultrasound date. This “redating” affects only about 10% of pregnancies but can have significant clinical implications for preterm or post-term management.

Trimester Breakdown: What to Expect

Pregnancy is divided into three trimesters, each with distinct developmental milestones and prenatal care priorities.

First Trimester: Weeks 1–12

This is when the embryo develops most of its major organ systems. The heart begins beating between weeks 6 and 7 — detectable by transvaginal ultrasound as early as 6 weeks. By week 12, most miscarriage risk has passed; first-trimester loss rates drop from approximately 10–20% in the general population to under 2% after a heartbeat is confirmed.

Second Trimester: Weeks 13–27

Often called the “comfortable trimester.” Morning sickness typically resolves, energy returns, and the pregnancy begins to show. The anatomy scan at 18–20 weeks is the most comprehensive ultrasound of the pregnancy, checking fetal structure and often revealing biological sex. Fetal viability — the gestational age at which a baby can potentially survive outside the womb with medical support — is generally recognized at 24 weeks.

Third Trimester: Weeks 28–40

The fetus gains most of its weight and the lungs mature during the final stretch. A pregnancy is considered “early term” at 37–38 weeks and “full term” from 39 to 40 weeks. According to CDC National Vital Statistics data (2022), the average gestational age at birth in the United States is approximately 38.9 weeks, and the preterm birth rate (before 37 weeks) was 10.4%.

Top 6 Key Pregnancy Milestones by Week

1. Week 6–7: Fetal heartbeat detectable via transvaginal ultrasound
2. Week 12: End of first trimester; miscarriage risk drops significantly
3. Week 18–20: Anatomy scan; biological sex often visible
4. Week 24: Viability threshold — survival possible with intensive NICU support
5. Week 28: Third trimester begins; survival rate with NICU care exceeds 90%
6. Week 39–40: Full-term; lowest risk of respiratory complications at birth

These milestones are averages. Individual pregnancies vary, and your OB-GYN will guide timing of specific screenings based on your personal health history.

Why Due Dates Are Estimates, Not Deadlines

Several biological factors make the EDD inherently imprecise:

• Ovulation timing varies: Cycles range from 21 to 35 days, shifting conception by up to two weeks from the assumed day-14 model.
• Implantation timing varies: After fertilization, the embryo takes 6 to 12 days to implant, adding another source of variation.
• Individual gestational length differs: A 2013 study published in Human Reproduction found that spontaneous labor onset varied by as much as 37 days among healthy pregnancies, even after controlling for LMP and ultrasound dating.

This is why clinicians speak of a “due window” rather than a due date. Deliveries between 39 weeks 0 days and 40 weeks 6 days are considered full-term; 37–38 weeks is early-term; 41 weeks is late-term; and 42 weeks or beyond is post-term.

Post-Term Pregnancy: What Happens After 42 Weeks

A pregnancy that continues past 42 completed weeks is post-term. Post-term pregnancies carry increased risks for both the baby and mother, including:

• Placental insufficiency (the placenta begins to age and function less effectively)
• Meconium aspiration syndrome (baby inhales stool in the amniotic fluid)
• Macrosomia (larger-than-average baby, increasing delivery complications)
• Increased cesarean delivery rate

According to ACOG guidelines, most providers offer or recommend labor induction between 41 weeks 0 days and 42 weeks 0 days for uncomplicated pregnancies. Delivery should not routinely extend beyond 42 weeks 0 days.

A 2018 study in the New England Journal of Medicine (the ARRIVE trial) found that elective induction at 39 weeks was associated with a lower cesarean delivery ratecompared to expectant management — challenging the previous assumption that waiting for spontaneous labor was always preferable.

Frequently Asked Questions

How is a pregnancy due date calculated?

Pregnancy due dates are calculated using Naegele's Rule: add 7 days to the first day of your last menstrual period, subtract 3 months, then add 1 year. This produces an EDD of approximately 40 weeks (280 days) from the LMP. First-trimester ultrasound is often more accurate for women with irregular cycles.

How accurate is a pregnancy due date?

Only about 5% of babies are born on their exact due date, per ACOG. Roughly 80% of births occur within two weeks of the EDD. The US average gestational age at birth is 38.9 weeks (CDC, 2022), meaning most babies arrive before the 40-week mark.

What is the difference between due date from LMP and due date from conception?

The LMP method adds 280 days to your last period's start date. The conception method adds 266 days to the fertilization date. Since ovulation typically happens 14 days after the LMP, both methods usually give the same EDD when cycles are regular. Irregular cycles make the conception method more precise.

When is ultrasound dating more accurate than LMP for calculating a due date?

First-trimester ultrasound before 14 weeks is the most accurate dating method. Crown-rump length (CRL) measurements are accurate to within 5 to 7 days. After 20 weeks, ultrasound accuracy drops to plus or minus 2 to 3 weeks, making late-pregnancy scans unreliable for revising established due dates.

What happens if you go past your due date?

A pregnancy is post-term after 42 completed weeks. Risks include placental insufficiency and meconium aspiration. Most providers discuss induction between 41 and 42 weeks. ACOG recommends delivery no later than 42 weeks 0 days for uncomplicated pregnancies.

What are the key pregnancy milestones by week?

Heartbeat detectable at 6 to 7 weeks. First trimester ends at 12 weeks. Anatomy scan at 18 to 20 weeks often reveals biological sex. Viability threshold at 24 weeks. Third trimester begins at 28 weeks. Early full-term at 37 weeks. Full-term window is 39 to 40 weeks.

Related Guides and Tools

• BMI Calculator Guide — understanding body mass index during and after pregnancy
• Water Intake Calculator Guide — hydration needs change significantly during pregnancy
• Calorie Burned Calculator Guide — how exercise needs shift across trimesters
• Due Date Calculator — calculate your EDD from LMP or conception date