Due Date Calculator: How Accurate Is Your Due Date Really?

Your due date feels like everything. It's the day you circle on the calendar, count down to, plan around. But here's the reality check most pregnancy books gloss over:

Only about 5% of babies are born on their exact due date. Your due date is better understood as a due "window"—most babies arrive sometime between 37 and 42 weeks of pregnancy. The standard 40-week calculation gives you a target, but your baby will arrive when they're ready.

I learned this firsthand with both of my pregnancies. My first due date was set for October 12th based on my LMP, then moved to October 8th after my 8-week ultrasound -- and my daughter arrived on October 17th, a full 9 days past the adjusted date. With my second, the due date was March 22nd and he came at 38 weeks and 5 days on March 15th. Two pregnancies, two completely different patterns, and neither baby cared about the date circled on my calendar. That real-world unpredictability is exactly what this guide addresses.

Use our Due Date Calculator to estimate your delivery date using multiple methods.

Let's explore how due dates are calculated, what affects their accuracy, and what the statistics actually show about when babies arrive.

How Due Dates Are Calculated

There are several methods for calculating an estimated due date (EDD), each with different levels of accuracy.

Method 1: Last Menstrual Period (LMP)

The most traditional method uses Naegele's Rule, developed in the early 1800s:

Formula: Due Date = First day of LMP + 280 days (40 weeks)

Simplified method:

1. Take the first day of your last period
2. Add 7 days
3. Subtract 3 months
4. Add 1 year (if applicable)

Example:

• LMP: March 10, 2026
• Add 7 days: March 17
• Subtract 3 months: December 17
• Due date: December 17, 2026

Our Due Date Calculator performs this calculation instantly.

Adjusted formula for different cycle lengths: Due Date = LMP + 280 + (Actual Cycle Length - 28)

Method 2: Ultrasound Dating

First-trimester ultrasounds provide the most accurate dating, measuring the embryo's crown-rump length (CRL).

According to the American College of Obstetricians and Gynecologists (ACOG), dating accuracy by gestational age:

Why early is more accurate: In the first trimester, all embryos grow at nearly identical rates. Later in pregnancy, individual variation in baby size makes dating less precise.

Method 3: Conception Date

If you know exactly when you conceived (common with IVF or precise ovulation tracking):

Formula: Due Date = Conception Date + 266 days (38 weeks)

This method is highly accurate for IVF pregnancies where fertilization date is known precisely.

Method 4: IVF Transfer Date

For IVF patients, due dates are calculated from transfer date:

IVF due dates are among the most accurate because conception timing is precisely known.

How Accurate Are Due Dates?

Here's what the statistics actually show about when babies arrive:

Distribution of Delivery Times

Key insight: About 70% of babies arrive within 10 days of their due date. But that still leaves 30% who arrive earlier or later.

The "Due Date" Myth

The 40-week due date is based on averages, but human pregnancy length naturally varies:

• Average first pregnancy: 41 weeks 1 day
• Average subsequent pregnancies: 40 weeks 3 days
• Individual variation: 37-42 weeks is considered normal range

A study published in Human Reproduction found that pregnancy length varies naturally by up to 5 weeks between women, even when conception date is precisely known.

Factors That Affect Due Date Accuracy

1. Cycle Regularity

Irregular cycles make LMP dating less accurate:

Women with PCOS, recent hormonal contraceptive use, or breastfeeding often have unpredictable ovulation, affecting LMP-based calculations.

2. First Pregnancy vs. Subsequent

If your first baby came at 41 weeks, subsequent pregnancies often follow similar patterns—but not always.

3. Mother's Age

Research shows maternal age affects pregnancy length:

4. Baby's Sex

Some studies suggest:

• Boys: Slightly more likely to go post-term
• Girls: Slightly more likely to arrive early

The difference is small (a few days on average) but statistically present.

5. Ethnicity

According to research from NIH, average pregnancy length varies somewhat by ethnicity:

• Average variation of about 1 week between different ethnic groups
• Due date calculations may be slightly less accurate for some populations

6. BMI

When Due Dates Change

Your due date may be adjusted during pregnancy. Here's when and why:

Reasons for Date Changes

1. Early ultrasound disagrees with LMP

   • If difference is significant, ultrasound date is usually adopted
   • This happens often with irregular cycles

2. Multiple ultrasounds suggest different timing

   • First trimester measurements take precedence
   • Later measurements assess growth, not dating

3. IVF with known conception date

   • May override LMP-based calculation
   • Most accurate dating method available

What Doesn't Change the Due Date

• Baby measuring "big" or "small" in third trimester
• Maternal weight gain
• Symptoms or feelings about timing
• Family history of early/late babies

Understanding "Term" Definitions

ACOG has specific definitions for pregnancy timing:

Why 39 Weeks Matters

Research shows babies born at 39+ weeks have better outcomes:

• Lower respiratory issues
• Better temperature regulation
• Improved feeding
• Reduced NICU admissions

This is why elective deliveries before 39 weeks are discouraged without medical indication.

What Happens When You Go Past Your Due Date

Going past your due date is common—especially for first pregnancies. Here's what to expect:

40-41 Weeks

• Additional monitoring may begin
• Non-stress tests (NST) to check baby's heart rate
• Amniotic fluid checks
• Discussion of induction timing

41+ Weeks

At this point, most providers recommend:

• Frequent monitoring (1-2 times per week)
• Discussion of induction benefits
• Consideration of cervical ripeness

42+ Weeks

According to ACOG, most providers recommend delivery by 42 weeks due to:

• Increased risk of stillbirth
• Placental aging
• Larger baby size
• Meconium in amniotic fluid

Induction and Due Dates

Reasons for Induction

Medical indications:

• Post-term pregnancy (42+ weeks)
• Preeclampsia
• Gestational diabetes
• Low amniotic fluid
• Placental issues
• Baby growth concerns

Elective induction (after 39 weeks):

• Maternal preference
• Logistics (distance, childcare)
• History of rapid labor
• Mental health considerations

The ARRIVE Study

A major study (ARRIVE trial) found that elective induction at 39 weeks for low-risk first-time mothers:

• Did NOT increase cesarean rates
• May have slightly reduced cesarean risk
• Had no adverse neonatal outcomes

This has shifted practice for some providers to offer elective induction at 39 weeks.

Making Peace with Due Date Uncertainty

Practical Tips

For planning:

• Don't schedule important events in the 2 weeks before or after your due date
• Have baby essentials ready by 37 weeks
• Prepare work transitions for a range of dates

For mental health:

• Focus on a "due range" rather than a single day
• Avoid telling everyone your exact due date
• Have plans for the waiting period

For physical preparation:

• Be ready for labor from 37 weeks
• Know your hospital route and backup routes
• Have bags packed by 36-37 weeks

Signs Labor Is Approaching

While the due date provides a general timeframe, your body gives signals that labor is near:

Days to Weeks Before

• Lightening: Baby drops lower into pelvis
• Increased Braxton Hicks: More frequent practice contractions
• Cervical changes: Effacement and early dilation
• Nesting instinct: Sudden urge to prepare

Hours to Days Before

• Bloody show: Mucus plug release
• Loose stools: Body clearing out
• Low back pain: Persistent and increasing
• Water breaking: Only happens first in about 15% of labors

True Labor Signs

• Regular contractions: Getting stronger, longer, closer
• 5-1-1 rule: 5 minutes apart, 1 minute long, for 1 hour
• Can't talk through contractions: Intensity requires focus
• Progress: Contractions don't stop with rest or position change

See our Complete Pregnancy Journey Guide for more on labor signs.

Alternative Due Date Theories

Nichols Rule

An alternative to Naegele's Rule that adds 15 days instead of 7, acknowledging that many pregnancies go past 40 weeks. This gives a due date 8 days later than standard.

Mittendorf-Williams Rule

Adjusts for:

• First pregnancy vs. subsequent
• Maternal race Gives different calculations for different populations.

The 10-Month Calendar

Some cultures and practitioners count pregnancy as 10 lunar months (40 weeks), which can feel more accurate than saying "9 months."

None of these alternatives have replaced Naegele's Rule in standard practice, but they highlight the inherent imprecision of due date calculations.

Frequently Asked Questions

How accurate is my due date?

Your due date is an estimate, not a precise prediction. About 5% of babies arrive on their exact due date, but about 70% arrive within 10 days of it. First-trimester ultrasound dating is most accurate (±5-7 days), while LMP dating is less precise, especially with irregular cycles.

Can my due date be wrong?

Yes. LMP-based due dates can be off if you have irregular cycles, ovulated later than expected, or aren't sure of your period dates. Ultrasound dating can also have a margin of error (especially later in pregnancy). The combination of LMP and first-trimester ultrasound gives the most reliable estimate.

Why did my doctor change my due date?

Due dates are typically changed when a first-trimester ultrasound differs significantly from the LMP-based calculation. ACOG guidelines recommend changing the due date if ultrasound differs by more than 5-7 days in the first trimester. This usually means you ovulated earlier or later than assumed.

What if I go past my due date?

Going past your due date is common, especially for first pregnancies. Most providers increase monitoring after 40 weeks and discuss induction options. By 41-42 weeks, induction is usually recommended due to increased risks with post-term pregnancy.

Can I make my baby come on my due date?

You cannot reliably make labor start on a specific date. While some activities (walking, sex, nipple stimulation) may potentially encourage labor when your body is already ready, there's no proven way to trigger labor at a desired time. Your baby will come when they're ready.

Should I trust LMP or ultrasound dating?

First-trimester ultrasound is generally more accurate than LMP dating. However, if your LMP and early ultrasound agree closely, that's a strong confirmation. If they differ significantly, the ultrasound date is usually adopted. For IVF pregnancies, the transfer date calculation is most accurate.

Why do some women always deliver early or late?

Individual physiology plays a role. Some women's bodies tend toward shorter pregnancies, while others consistently go longer. Genetics, cervical length, and hormonal patterns may contribute. If you consistently deliver early or late, future pregnancies may follow the pattern—but not always.

Does my due date affect when I can be induced?

Yes. Elective induction (without medical indication) is generally not offered before 39 weeks due to baby's development needs. Medical inductions can happen earlier when necessary. After 39 weeks, many providers offer elective induction; after 41-42 weeks, induction is typically recommended.

Related Articles

• Complete Pregnancy Journey: From Conception to Due Date — Full timeline from conception to birth
• How to Track Ovulation — Accurate ovulation dating improves due date accuracy
• When Did I Conceive? — Calculate conception from your due date
• Implantation Guide — Understanding early pregnancy timing
• Pregnancy Week by Week — What to expect as you approach your due date

Related Calculators

• Due Date Calculator — Calculate your due date multiple ways
• Pregnancy Calculator — Track your pregnancy week by week
• Conception Calculator — Work backwards from your due date
• Ovulation Calculator — Understand your cycle timing
• Implantation Calculator — Estimate implantation timing

---

This article provides general information for educational purposes. Due dates are estimates, and every pregnancy is unique. Discuss any concerns about your due date or delivery timing with your healthcare provider.