Pregnancy Due Date: How It Is Calculated and What It Really Means

Updated April 2026 · By the FertilityCalcs Team

Your estimated due date is one of the first pieces of information you receive in pregnancy, and it shapes your entire timeline of prenatal care, leave planning, and mental preparation. Yet only about 5 percent of babies are born on their due date, and the calculation method itself has limitations that most expectant parents do not understand. This guide explains how due dates are determined, when and why they change, and what happens when your baby does not follow the calendar.

How Due Dates Are Calculated

The standard due date calculation uses Naegele rule: take the first day of your last menstrual period (LMP), add 7 days, subtract 3 months, and add 1 year. This assumes a 28-day cycle with ovulation on day 14 and produces an estimated due date (EDD) of 280 days (40 weeks) from the LMP. Online due date calculators use this same formula.

The problem is that this formula assumes ovulation occurred on day 14 of a 28-day cycle, which is only true for a fraction of women. If your cycles are longer or shorter than 28 days, or if you ovulated later than day 14, the LMP-based date can be off by a week or more. Women who know their exact ovulation date (through tracking or fertility treatment) have more accurate dating from the start.

• Naegele rule: LMP + 280 days = estimated due date
• Assumes 28-day cycle with day 14 ovulation
• Only 5% of babies born on the exact due date
• Full term range: 39 weeks 0 days to 40 weeks 6 days
• Most babies arrive between 38 and 42 weeks

Dating Ultrasounds: Accuracy and Timing

First trimester ultrasound between 8 and 13 weeks is the most accurate method for determining gestational age and due date. At this stage, embryo size correlates very closely with age because growth rate varies little between individuals. Accuracy is plus or minus 5 to 7 days. If the ultrasound date differs from the LMP date by more than 7 days, the ultrasound date is typically used.

Second trimester ultrasounds (14 to 20 weeks) are accurate to plus or minus 10 to 14 days. Third trimester ultrasounds are accurate to plus or minus 21 days, which is too imprecise for dating purposes. This is why early dating ultrasounds are so valuable, especially for women with irregular cycles or uncertain LMP dates.

Why Due Dates Change

Due dates are revised when ultrasound measurements disagree with LMP-based dating by more than the accepted margin (7 days in the first trimester, 10-14 days in the second). This does not mean the baby is growing abnormally; it means the original estimate was based on an incorrect assumption about ovulation timing.

Once a due date is established by first trimester ultrasound, it should not be changed based on later ultrasound measurements. Later in pregnancy, size differences reflect individual growth variation, not dating errors. A baby measuring large at 32 weeks does not mean the due date was wrong; it means the baby is growing at a faster rate.

What Full Term Really Means

The American College of Obstetricians and Gynecologists defines pregnancy timing categories: early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and post-term is 42 weeks 0 days and beyond.

These distinctions matter medically. Babies born at 37 weeks have higher rates of respiratory problems and NICU admission than those born at 39 weeks. Elective delivery before 39 weeks without a medical indication is discouraged by all major obstetric organizations. If you are scheduling an induction or cesarean, 39 weeks is the earliest recommended timing for non-medical reasons.

• 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
• Elective delivery not recommended before 39 weeks

Going Past Your Due Date

Approximately 50 percent of pregnancies last beyond 40 weeks, and 10 percent go past 41 weeks. Going past your due date is normal and common, especially for first-time mothers. However, risks begin to increase after 41 weeks, including larger baby size, decreased amniotic fluid, and placental aging.

Most providers recommend induction between 41 and 42 weeks based on individual risk factors. The ARRIVE trial showed that induction at 39 weeks in low-risk first-time mothers reduced cesarean rates without increasing adverse outcomes, but this remains a personal and medical decision. If you go past your due date, your provider will typically increase monitoring with non-stress tests and fluid checks.