How is the due date calculated?

The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation at day 14. Only about 5% of babies are born on their exact due date — most deliver within 2 weeks before or after.

What if my cycle is longer or shorter than 28 days?

If your cycle is regularly longer or shorter than 28 days, your ovulation date differs from the assumed day 14. Adjustments can be made: add or subtract the difference from 28 days. Ultrasound dating in the first trimester is often more accurate for irregular cycles.

What are the three trimesters?

First trimester: weeks 1–12 (organ formation). Second trimester: weeks 13–27 (rapid growth, movements felt). Third trimester: weeks 28–40 (final development, preparation for birth). Each trimester has different milestones and prenatal care recommendations.

How accurate is the due date?

Naegele's rule is accurate to ±2 weeks for women with regular 28-day cycles. First-trimester ultrasound is the most accurate dating method (±5–7 days). Only about 5% of babies arrive on the exact due date. A 'term' pregnancy is 37–42 weeks.

What is Naegele's rule?

Naegele's rule, developed by German obstetrician Franz Naegele in the early 1800s, calculates the due date by adding 280 days to the first day of the last menstrual period. An alternative method: subtract 3 months from the LMP and add 7 days. It assumes ovulation on cycle day 14.

Why is pregnancy counted from LMP instead of conception?

Historically, women knew their last period date but not their conception date. This convention persists because LMP is reliably known while ovulation timing varies. 'Gestational age' starts from LMP even though conception occurs about 2 weeks later.

When is the best time for dating ultrasound?

First-trimester ultrasound between 7-13 weeks, measuring crown-rump length (CRL), is most accurate for dating (±5-7 days). Second-trimester dating is accurate to ±2 weeks. Third-trimester dating is inaccurate (±3-4 weeks) due to growth variation.

What does 'term' mean in pregnancy?

Early term: 37 weeks 0 days to 38 weeks 6 days. Full term: 39 weeks 0 days to 40 weeks 6 days. Late term: 41 weeks 0 days to 41 weeks 6 days. Post-term: 42 weeks 0 days and beyond. Full term (39-40 weeks) is ideal for delivery when no complications exist.

Can my due date change after the first trimester?

ACOG recommends that once dating is established in the first trimester, it should not be changed. Fetal growth variation in later pregnancy doesn't indicate dating error — it reflects individual growth patterns. Changing due dates late in pregnancy can lead to inappropriate interventions.

What if I conceived through IVF?

IVF pregnancies have known conception dates. Add 266 days to the egg retrieval/fertilization date for your due date. Alternatively, add 263 days from a day-3 embryo transfer or 261 days from a day-5 blastocyst transfer.

How do I calculate due date from conception?

If you know your conception or ovulation date, add 266 days (38 weeks) for your due date. This is 14 days less than the 280-day LMP method because gestational age includes 2 weeks before conception. Ovulation kits or tracked BBT can confirm ovulation date.

What is the difference between gestational age and fetal age?

Gestational age counts from LMP (approximately 40 weeks at birth). Fetal age (embryonic/conceptional age) counts from fertilization (approximately 38 weeks at birth). Medical records use gestational age. The difference is about 2 weeks.

Why might I deliver early or late?

First pregnancies average 41 weeks. Prior preterm delivery increases preterm risk. Factors like multiples, cervical insufficiency, preeclampsia (higher risk with elevated [ASCVD Risk](/tools/ascvd-risk) or obesity [check [BMI](/tools/bmi-calculator)]), or placental issues can cause early delivery. Post-term pregnancy (42+ weeks) is more common in first pregnancies and some families.

Should I plan based on my due date?

Plan for a window of 37-42 weeks rather than a specific date. Don't schedule critical events the week of your due date. Have your hospital bag packed by 36 weeks. Most providers won't let you travel far after 36 weeks. Plan work handoffs for 38 weeks just in case.

What if I want to choose my delivery date?

Elective induction at 39+ weeks or scheduled cesarean is possible but should be discussed with your provider. ACOG recommends against non-medical induction before 39 weeks due to newborn risks. Your provider will consider your [Bishop Score](/tools/bishop-score) and circumstances.

Online Medical Tools — Due Date Calculator

Printed on 8/14/2026

For informational purposes only. This is not medical advice.

Obstetrics

Due Date Calculator

This pregnancy due date calculator uses Naegele's rule to estimate your expected delivery date (EDD) from the first day of your last menstrual period (LMP). It adds 280 days (40 weeks) to the LMP date and shows your current gestational age, trimester, and days remaining. Naegele's rule assumes a 28-day menstrual cycle with ovulation on day 14.

Formula: EDD = LMP + 280 days (Naegele's Rule)

Last Menstrual Period (LMP)

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How It Works

Enter your last menstrual period date

Select the first day of your most recent period. This is the starting point for all pregnancy dating calculations, even though conception occurs about two weeks later.

Calculate using Naegele's rule

The calculator adds 280 days (40 weeks) to your LMP date. This standard formula, developed in the 1800s, remains the foundation of pregnancy dating worldwide.

Review your results

See your estimated due date, current gestational age in weeks and days, trimester, and countdown to delivery. Use this to plan prenatal visits and prepare for baby's arrival.

Who Uses the Due Date Calculator

First-Time Parents

Newly pregnant couples

Calculate your due date immediately after a positive pregnancy test to begin planning prenatal care, share news with family, and prepare your timeline.

OB/GYN Clinicians

Obstetricians, midwives, nurses

Quickly establish dating at the first prenatal visit to schedule appropriate screening tests and plan the care timeline before ultrasound confirmation.

Pregnancy Planners

Couples planning conception

Calculate potential due dates based on planned conception timing to align pregnancy with work schedules, seasons, or personal preferences.

Working Parents

Employees planning leave

Determine your expected delivery window to plan maternity/paternity leave, project handoffs, and workplace coverage arrangements.

Experienced Mothers

Second or third pregnancies

Track a new pregnancy alongside caring for older children, coordinating school schedules and sibling preparation with your due date.

High-Risk Pregnancy Teams

Maternal-fetal medicine specialists

Establish baseline dating to plan intensified monitoring schedules, specialist consultations, and potential early delivery timing. Assess pre-pregnancy health with [BMI Calculator](/tools/bmi-calculator) and cardiovascular risk with [ASCVD Risk Calculator](/tools/ascvd-risk).

Pro Tips

Record your LMP before you forget

The first day of your last period is the most important date in pregnancy dating. Write it down or log it in an app as soon as you miss your period — memory fades quickly and accurate dating affects your entire care timeline.

First-trimester ultrasound is the gold standard

While LMP-based dating is useful, a crown-rump length measurement between 7-13 weeks is accurate to within 5-7 days. If ultrasound dating differs from LMP by more than 7 days, clinicians will typically use the ultrasound date.

Adjust for irregular cycles

If your cycles are regularly longer than 28 days, add the extra days to your due date. For example, with a 35-day cycle, your actual ovulation is about 7 days later than assumed, so your due date is 7 days later than Naegele's rule predicts.

Understand what 'due date' really means

Your EDD is the middle of a delivery window, not a deadline. Only 5% of babies arrive on their exact due date. About 80% deliver between 38-42 weeks. Think of it as an 'estimate' rather than an appointment.

Plan prenatal tests around gestational age

Key tests are time-sensitive: first-trimester screening at 11-13 weeks, anatomy scan at 18-22 weeks, glucose tolerance test at 24-28 weeks, and Group B Strep screening at 36-37 weeks. Know your gestational age to schedule correctly. For gestational diabetes management, track nutrition with [TDEE Calculator](/tools/tdee-calculator) and [Macro Calculator](/tools/macro-calculator).

Track by weeks and days, not months

Medical professionals count pregnancy in weeks and days (e.g., '28 weeks 4 days'), not months. This precision matters for clinical decisions. When asked 'how far along,' give weeks rather than months.

Use conception date if you know it

If you know your exact ovulation or conception date (from tracking or IVF), add 266 days to get your due date. This is more accurate than LMP for women who tracked ovulation precisely.

Discuss dating discrepancies with your provider

If your calculated due date differs significantly from ultrasound dating, ask your provider which date they're using and why. Consistent dating throughout pregnancy prevents confusion about timing decisions.

Multiple pregnancies deliver earlier

Twins average delivery at 36-37 weeks, triplets at 32-34 weeks. If you're carrying multiples, your 'due date' is a reference point, but expect delivery earlier than the standard 40 weeks. Increased nutritional needs require careful tracking with [Calorie Calculator](/tools/calorie-calculator) and [Protein Intake Calculator](/tools/protein-intake).

Post-term pregnancy is beyond 42 weeks

If you reach 41 weeks without delivery, your provider will discuss induction. True post-term pregnancy (beyond 42 weeks) carries increased risks and is rarely allowed to continue without intervention in modern obstetric care.

Common Questions About Your Results

Evidence-Based Methodology

Naegele's rule dates to 1812 and remains the standard for initial pregnancy dating per ACOG guidelines. First-trimester ultrasound (crown-rump length) is accurate to ±5-7 days and is the gold standard when LMP is uncertain (Papageorghiou et al., Lancet 2014). Only 5% of births occur on the exact due date, with 80% occurring within ±11 days (Mongelli et al., BJOG 1996).

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

Your estimated due date (EDD) represents the date at which you will be 40 weeks pregnant, which is considered full term. However, only about 5% of babies are born on their exact due date. A normal, full-term delivery can occur anywhere between 37 and 42 weeks of gestation.

The calculator also shows your current gestational age in weeks and days, along with your trimester. First trimester (weeks 1–12) is a critical period of organ formation. Second trimester (weeks 13–27) involves rapid fetal growth and is when most women begin to feel fetal movement. Third trimester (weeks 28–40) involves final organ maturation and preparation for delivery.

When to Use This Tool

Use this calculator early in pregnancy to estimate your due date and track your gestational progress. It is most appropriate for women with regular menstrual cycles who can reliably identify the first day of their last menstrual period.

The estimated due date is used to schedule prenatal visits, plan routine screening tests (e.g., first-trimester screening at 11–13 weeks, anatomy scan at 18–22 weeks, glucose tolerance test at 24–28 weeks), and guide clinical decisions about timing of delivery.

Limitations

Naegele's rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. For women with irregular cycles, longer or shorter cycles, or uncertain LMP dates, the estimate may be off by days to weeks. In these situations, first-trimester ultrasound dating (crown-rump length measurement between 7–13 weeks) is more accurate, with a margin of error of only ±5–7 days.

The calculator does not account for factors that can affect delivery timing, such as multiple gestation (twins/triplets tend to deliver earlier), maternal medical conditions (preeclampsia, gestational diabetes), or history of preterm or post-term delivery. It provides a statistical estimate — not a prediction of when labor will actually begin.

Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.

Changelog

April 21, 2026 · trust-baseline
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