How Pregnancy Due Date Calculations Work

Learning that you are pregnant brings an immediate question: when will the baby arrive? The estimated due date, also called the expected date of delivery, gives you a timeline for prenatal care, workplace planning, and emotional preparation. Understanding how this date is calculated helps you make sense of the numbers your healthcare provider shares and set realistic expectations for delivery timing.

In this guide, you will learn the most widely used due date formula, how ultrasound measurements refine the estimate, what the trimester timeline looks like, and why only about 4% of babies arrive on the exact predicted date.

What Is an Estimated Due Date?

An estimated due date is the projected date when a pregnancy reaches 40 weeks of gestation, measured from the first day of the last menstrual period. Healthcare providers use this date to schedule prenatal visits, order time-sensitive screening tests, and monitor fetal growth against expected milestones. The term "estimated" is important because the due date represents the statistical midpoint of a likely delivery window, not a guarantee of when labor will start.

Full-term pregnancy spans from 39 weeks and 0 days through 40 weeks and 6 days. Early term is 37 weeks through 38 weeks and 6 days, and late term covers 41 weeks through 41 weeks and 6 days. Babies born before 37 completed weeks are considered preterm, while those still in the womb after 42 weeks are post-term. These classifications guide clinical decisions about interventions, monitoring frequency, and delivery planning.

Naegele's Rule Explained

The most common method for calculating a due date is Naegele's rule, named after the German obstetrician Franz Karl Naegele who popularized it in the early 1800s. Despite being over two centuries old, this formula remains the foundation of pregnancy dating worldwide because of its simplicity and reasonable accuracy for individuals with regular menstrual cycles.

In practice, the formula works as a three-step shortcut: take the first day of your last menstrual period, subtract 3 months, then add 7 days and 1 year. Both methods produce the same result, which is the date exactly 280 days after the start of the last period.

The Formula Step by Step

Suppose the first day of your last menstrual period was June 1, 2026. Using Naegele's rule:

1. Start with June 1, 2026
2. Subtract 3 months: March 1, 2026
3. Add 7 days: March 8, 2026
4. Add 1 year: March 8, 2027

The estimated due date is March 8, 2027. You can verify this by counting 280 days forward from June 1, which lands on the same date. This calculation assumes ovulation occurred on day 14 of a standard 28-day cycle, meaning conception happened approximately two weeks after the LMP.

Adjusting for Irregular Cycles

Naegele's rule assumes a 28-day menstrual cycle, but cycle lengths vary widely among individuals. If your cycle is consistently longer or shorter than 28 days, you can adjust the formula by adding or subtracting the difference from the standard 14-day ovulation point.

For example, if your average cycle is 35 days, ovulation likely occurs around day 21 rather than day 14. That is a 7-day difference, so you would add 7 days to the Naegele's rule result. If your cycle averages 25 days, ovulation may occur around day 11, so you would subtract 3 days from the standard estimate. This adjusted calculation is sometimes called the modified Naegele's rule.

Ultrasound Dating and Accuracy

While Naegele's rule provides a starting estimate, ultrasound measurements offer the most reliable pregnancy dating, particularly in the first trimester. The American College of Obstetricians and Gynecologists considers first-trimester ultrasound the most accurate method for confirming gestational age.

First Trimester Ultrasound

Between 8 and 13 weeks of gestation, the crown-rump length measurement of the embryo is highly predictive of gestational age. During this period, embryos grow at a remarkably consistent rate across all healthy pregnancies, which makes size a reliable indicator of age. A first-trimester ultrasound can estimate gestational age within plus or minus 5 to 7 days.

If the ultrasound-based estimate differs from the LMP-based due date by more than 5 days before 9 weeks, or by more than 7 days between 9 and 14 weeks, the due date is typically revised to match the ultrasound measurement. This recommendation comes directly from ACOG and helps ensure that prenatal care milestones and screening tests are properly timed.

Second and Third Trimester Accuracy

As the table shows, ultrasound accuracy decreases significantly as pregnancy progresses. By the third trimester, individual fetal growth patterns create substantial size variation, and measurements can be off by up to three weeks. This is why confirming the due date early in pregnancy is so important for accurate prenatal care planning.

Trimester Timeline Overview

Pregnancy is divided into three trimesters, each spanning approximately 13 weeks. Understanding this timeline helps you anticipate key milestones and prepare for upcoming prenatal appointments.

Most prenatal visits occur monthly during the first and second trimesters, then increase to every two weeks around 28 to 36 weeks, and become weekly from 36 weeks until delivery. This schedule allows your healthcare provider to track growth, screen for complications, and adjust your care plan as needed.

Practical Due Date Examples

Example 1: Regular 28-Day Cycle

Maria has a regular 28-day menstrual cycle. Her last period started on January 15, 2026. Using Naegele's rule:

• LMP: January 15, 2026
• Subtract 3 months: October 15, 2025
• Add 7 days and 1 year: October 22, 2026
• Estimated due date: October 22, 2026

At her 10-week ultrasound, the crown-rump length measures consistent with 10 weeks and 2 days, which matches the LMP-based estimate within the 7-day threshold. Her due date remains October 22. Maria can expect to enter the second trimester around April 9, the third trimester around July 2, and reach full term on October 8.

Example 2: Longer 35-Day Cycle

Priya has a consistent 35-day cycle. Her last period started on March 1, 2026. The standard Naegele's rule calculation gives:

• LMP: March 1, 2026
• Standard calculation: December 6, 2026
• Cycle adjustment: 35 - 28 = 7 additional days
• Modified due date: December 13, 2026

Because Priya ovulates around day 21 instead of day 14, her actual conception happened about a week later than the standard formula assumes. Her first-trimester ultrasound at 11 weeks confirms a gestational age that aligns with the adjusted December 13 date. Without this cycle length correction, prenatal screening tests could have been scheduled too early relative to actual fetal development.

Example 3: IVF Pregnancy

Aisha conceived through in vitro fertilization. A Day 5 blastocyst was transferred on April 10, 2026. For IVF pregnancies, the calculation is different:

• Transfer date: April 10, 2026
• Gestational age at transfer: 2 weeks and 5 days (for Day 5 embryo)
• Equivalent LMP: March 22, 2026 (transfer date minus 19 days)
• Due date: 280 days from equivalent LMP = December 27, 2026

IVF pregnancies have the most accurate dating because the exact moment of fertilization is known. ACOG recommends using the ART-derived gestational age as the primary dating method, superseding even first-trimester ultrasound measurements. Aisha's healthcare provider documents the due date as December 27, and no revision is needed regardless of ultrasound findings.

Factors That Can Shift Your Due Date

Several factors can cause the actual delivery date to differ significantly from the estimated due date. Understanding these variables helps set realistic expectations.

• Irregular menstrual cycles. Cycles that vary in length from month to month make LMP-based dating unreliable. Individuals with polycystic ovary syndrome, recent hormonal contraceptive use, or stress-related irregularities benefit most from early ultrasound dating.
• First pregnancy. First-time pregnancies tend to go slightly longer than subsequent pregnancies. Research suggests first babies average about 5 days past the due date, while second and later babies tend to arrive closer to or slightly before the estimated date.
• Maternal age. Individuals over 35 are more likely to have slightly longer gestations, though the difference is typically small and managed through closer monitoring in the third trimester.
• Multiple gestation. Twin and higher-order pregnancies frequently deliver earlier than singleton pregnancies. The average twin delivery occurs around 36 to 37 weeks, while triplets average around 32 to 34 weeks.
• Medical conditions. Gestational diabetes, preeclampsia, and other pregnancy complications may lead to earlier delivery, either through spontaneous preterm labor or planned induction for medical safety.

Tips for Understanding Your Due Date

Think of your due date as a due window rather than a due day. Most healthcare providers consider any delivery between 39 and 41 weeks to be ideal timing. Planning your work leave, childcare arrangements, and hospital preparation around a two-week range before and after the due date gives you practical flexibility.

Track your menstrual cycles before conception if possible. Having accurate data on your average cycle length and the date of your last period gives your healthcare provider the best starting information. Period tracking apps can help maintain a reliable record over several months.

Attend your first-trimester ultrasound appointment. This early scan provides the most accurate gestational age assessment and helps calibrate all future prenatal care scheduling. If your LMP date is uncertain or your cycles are irregular, this ultrasound becomes the primary dating tool for the entire pregnancy.

Discuss any due date discrepancies with your provider. If your LMP-based date and ultrasound-based date differ, ask your healthcare team which date they are using for your care and why. Understanding the reasoning helps you track your own timeline and know what to expect at each visit.

Common Mistakes to Avoid

• Treating the due date as a deadline. Feeling anxious when the due date passes is natural, but remember that delivery any time between 39 and 41 weeks is considered full term. Going past the due date by several days is completely normal and does not automatically indicate a problem.
• Using the wrong date for LMP. The calculation uses the first day of bleeding, not the last day of your period. Using the wrong starting point can shift the entire estimate by a week or more.
• Relying on late-pregnancy ultrasounds for dating. Third-trimester ultrasounds are useful for monitoring growth but are unreliable for pregnancy dating because fetal size varies too much at this stage. Always rely on the earliest available measurement for your due date.
• Ignoring cycle length variation. Applying the standard 28-day formula to a 35-day cycle adds a week of error to the estimate. Always inform your provider about your typical cycle length so adjustments can be made.
• Comparing your timeline to others. Every pregnancy is unique. Comparing due date accuracy or delivery timing with friends or family creates unnecessary stress. Focus on your own prenatal care schedule and trust your healthcare team's guidance.

Last updated: February 23, 2026