How Pregnancy Due Dates Are Calculated

One of the first questions after a positive pregnancy test is “when is the baby due?” Doctors have been estimating due dates for centuries, and while the methods have become more precise, the underlying biology means every estimate is exactly that: an estimate. Understanding how the calculation works helps set realistic expectations and prepares you for the weeks ahead.

This guide explains the main methods used to determine a due date, breaks down the trimester system, and addresses the factors that affect accuracy.

Naegele’s Rule: The Standard Calculation

The most widely used method for calculating a due date is Naegele’s rule, developed by German obstetrician Franz Naegele in the early 1800s. The formula is simple:

Take the first day of your last menstrual period (LMP), add one year, subtract three months, and add seven days.

In practice, this amounts to adding 280 days (40 weeks) to the first day of your last period.

Example:

• Last menstrual period started: June 1, 2026
• Subtract three months: March 1, 2026
• Add one year and seven days: March 8, 2027
• Estimated due date: March 8, 2027

This formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. Since pregnancy is dated from the LMP rather than from conception, you are technically counted as “pregnant” during the two weeks before you actually conceive. Gestational age (how far along you are) includes those two weeks, which is why a “40-week” pregnancy really involves about 38 weeks of actual fetal development.

Adjustments for Irregular Cycles

Naegele’s rule works best for people with regular 28-day cycles. If your cycles are consistently longer or shorter, the estimated due date shifts accordingly.

For a 35-day cycle, ovulation likely occurs around day 21 instead of day 14. That is a full week later, which means the due date should be pushed forward by seven days. Conversely, a 24-day cycle suggests earlier ovulation and an earlier due date.

Modified formula: Add (cycle length minus 21) days to the standard Naegele’s date.

For a 35-day cycle: 35 - 21 = 14 extra days, so add 14 days to the Naegele’s date (or equivalently, add 294 days to the LMP instead of 280).

If your cycles are irregular or you are unsure of your last period date, an early ultrasound becomes the primary dating method.

Ultrasound Dating

Ultrasound measurements provide the most accurate way to confirm or adjust a due date, particularly when performed in the first trimester (before 14 weeks).

First Trimester Ultrasound (Most Accurate)

Between 6 and 13 weeks, the embryo is measured from crown to rump (CRL). At this stage, embryos grow at remarkably consistent rates regardless of genetics, so the measurement provides a reliable gestational age estimate accurate to within about five days.

If the ultrasound date differs from the LMP-based date by more than seven days, most practitioners will adjust the due date to match the ultrasound.

Second Trimester Ultrasound

Between 14 and 27 weeks, dating uses multiple measurements: head circumference, abdominal circumference, and femur length. Accuracy drops to within about 10 to 14 days because genetic variation in fetal size becomes more significant.

Third Trimester Ultrasound

After 28 weeks, ultrasound dating is accurate only to within about three weeks. At this stage, differences in fetal growth are largely genetic, and a larger or smaller baby doesn’t necessarily mean the due date is wrong. Third-trimester ultrasounds are rarely used to change a previously established due date.

IVF and Assisted Reproduction Dating

For pregnancies conceived through in vitro fertilization (IVF), the due date calculation is more precise because the exact date of fertilization (or embryo transfer) is known.

• Day 3 embryo transfer: Due date = transfer date + 263 days
• Day 5 embryo transfer (blastocyst): Due date = transfer date + 261 days

These calculations are equivalent to counting 38 weeks from fertilization, which aligns with the 40 weeks from LMP since LMP-based dating adds two weeks before conception.

The Trimester System

Pregnancy is divided into three trimesters, each covering roughly 13 weeks.

First Trimester (Weeks 1-13)

This period covers fertilization, implantation, and early organ formation. By the end of the first trimester, all major organs and systems have formed. Key milestones include:

• Week 4: Implantation; positive pregnancy test possible
• Week 6: Heartbeat detectable on ultrasound
• Week 8: All major organs are forming
• Week 12: Risk of miscarriage drops significantly

Second Trimester (Weeks 14-27)

Often called the most comfortable trimester, this is when organs mature and the fetus grows rapidly. Milestones include:

• Week 16: Sex may be visible on ultrasound
• Week 18-22: Anatomy scan performed; movement (quickening) felt
• Week 24: Viability milestone; survival outside the womb becomes possible with intensive medical support
• Week 27: Eyes open; sleep-wake cycles develop

Third Trimester (Weeks 28-40)

The final trimester focuses on growth, brain development, and preparation for birth:

• Week 28: Lungs begin producing surfactant
• Week 32: Bones harden (except skull, which stays flexible for birth)
• Week 36: Baby is considered “early term”
• Week 37: Lungs are typically mature
• Week 39-40: Full term

What “Full Term” Actually Means

The American College of Obstetricians and Gynecologists (ACOG) defines term pregnancy in specific categories:

• Early term: 37 weeks 0 days through 38 weeks 6 days
• Full term: 39 weeks 0 days through 40 weeks 6 days
• Late term: 41 weeks 0 days through 41 weeks 6 days
• Post term: 42 weeks 0 days and beyond

Only about 5% of babies arrive on their exact due date. The majority are born within a two-week window on either side. First-time mothers tend to deliver slightly later on average, while subsequent pregnancies may come a few days earlier.

Factors That Affect Due Date Accuracy

Several factors influence how close the actual birth date falls to the estimated due date:

• Uncertain LMP: If you’re unsure of the date of your last period, the baseline for Naegele’s rule is unreliable.
• Irregular cycles: Variation in cycle length directly affects when ovulation occurred.
• Maternal age: Some studies suggest older mothers carry slightly longer on average.
• First vs. subsequent pregnancies: First pregnancies tend to go a few days past the due date more often.
• Genetics: Family history of early or late deliveries can play a role.
• Multiple pregnancies: Twins and triplets typically arrive earlier, with the average twin delivery at 36 weeks.

Using a Due Date Calculator

Our Pregnancy Due Date Calculator applies Naegele’s rule with adjustments for your specific cycle length. Enter the first day of your last period and your average cycle length, and it returns your estimated due date along with a trimester timeline. Keep in mind that this is an estimate; your healthcare provider may adjust the date based on ultrasound findings.

Frequently Asked Questions

Why is pregnancy counted from the last menstrual period instead of conception?

Most people know the date their last period started but not the exact date of conception, since sperm can survive several days and fertilization may occur one to five days after intercourse. Dating from the LMP provides a consistent, easily identifiable starting point. It does mean the first two weeks of “pregnancy” occur before conception, which is why gestational age is always about two weeks ahead of actual fetal age.

Can my due date change during pregnancy?

Yes. If an early ultrasound shows a gestational age that differs from the LMP-based estimate by more than seven days, your provider will likely adjust the due date. Once a due date is established by first-trimester ultrasound, it generally stays the same even if later ultrasounds suggest slightly different measurements, since later scans reflect individual growth variations rather than dating errors.

What happens if I go past my due date?

Going past the due date is common, especially for first pregnancies. Most providers begin increased monitoring around 41 weeks and discuss induction options between 41 and 42 weeks. Post-term pregnancy (beyond 42 weeks) carries increased risks, so few providers recommend waiting beyond that point without intervention.

How accurate is a due date based on cycle length alone?

For people with regular cycles who are certain of their LMP date, Naegele’s rule (adjusted for cycle length) estimates the due date within about 10 days of actual delivery for the majority of pregnancies. Adding an early ultrasound confirmation improves accuracy to within about five days. No method can predict the exact day of birth.

Does the due date differ for twins or multiples?

The estimated due date is calculated the same way, but the expected delivery date is earlier. Twins average 36 weeks, and triplets average 32 weeks. Your provider will discuss a target delivery window specific to your pregnancy type, but the “due date” on your chart will still reflect the standard 40-week calculation.