How Pregnancy Due Dates Are Calculated

One of the first questions every expectant parent asks is: “When is my baby due?” The answer might seem straightforward, but the science behind estimating a due date involves a combination of historical rules, modern imaging, and a healthy dose of statistical reality. Understanding how your estimated due date (EDD) is determined can help you plan more confidently and set realistic expectations for those final weeks of pregnancy.

Naegele's Rule: The 280-Day Standard

The most widely used method for calculating a due date is called Naegele's Rule, named after the German obstetrician Franz Naegele who popularized it in the early 1800s. The formula is simple: take the first day of the mother's last menstrual period (LMP) and add 280 days, or 40 weeks. In practice, many providers use a shortcut version: subtract three months from the LMP date, then add seven days and one year.

For example, if your last period started on January 1, you would count back three months to October 1, add seven days to get October 8, and add one year — giving you an estimated due date of October 8 the following year. This calculation assumes a 28-day menstrual cycle with ovulation occurring on day 14, which is why it works well as a general estimate but may need adjustment for many women.

Why Pregnancy Is Counted from LMP, Not Conception

It often surprises people to learn that pregnancy dating starts from the last menstrual period rather than the date of conception. The reason is practical: most women know when their last period started, but the exact date of conception is much harder to pin down. Ovulation can vary by several days even in women with regular cycles, and sperm can survive in the reproductive tract for up to five days. By using the LMP as a consistent starting point, healthcare providers have a reliable anchor for tracking pregnancy progress across all patients.

This convention means that during the first two weeks of “pregnancy,” the woman is not actually pregnant yet — she has not ovulated or conceived. Actual fertilization typically happens around week two or three of the gestational timeline.

Gestational Age vs. Fetal Age

Because pregnancy is dated from the LMP, there are two different ways to describe how far along a pregnancy is. Gestational age counts from the first day of the last menstrual period and is the standard used in medical records, ultrasound reports, and when your doctor says you are “20 weeks pregnant.” Fetal age (also called embryonic age or conceptional age) counts from the actual date of fertilization and is roughly two weeks less than gestational age.

When you read about fetal development milestones, pay attention to which system is being used. A “12-week fetus” by gestational age has only been developing for about 10 weeks since conception. Most medical literature and pregnancy apps use gestational age, so that is the convention we follow here.

How Ultrasound Dating Works

While Naegele's Rule provides a quick estimate, ultrasound measurement is often more accurate, especially in the first trimester. During an early ultrasound (typically between 6 and 13 weeks), the technician measures the embryo from crown to rump (CRL). Because embryos grow at a remarkably consistent rate in early pregnancy, this measurement can estimate gestational age to within about five to seven days.

If the ultrasound date and the LMP-based date differ by more than seven days in the first trimester, providers will often adjust the due date to match the ultrasound. Later ultrasounds (second and third trimester) are less reliable for dating because individual growth variation increases — babies start growing at different rates based on genetics, nutrition, and other factors. A second-trimester ultrasound may be accurate to within 10 to 14 days, while a third-trimester scan can be off by as much as three weeks.

The Three Trimesters Explained

Pregnancy is divided into three trimesters, each lasting roughly 13 weeks. These divisions reflect distinct phases of fetal development and maternal changes.

First Trimester (Weeks 1–12)

The first trimester is the period of most rapid foundational development. By the end of week 12, all major organs and body systems have begun to form. The embryonic heart starts beating around week 5 to 6 and can usually be detected on ultrasound by week 6 or 7. This is also the trimester when many women experience morning sickness, fatigue, and other early pregnancy symptoms. The risk of miscarriage is highest during this period and drops significantly after week 12.

Second Trimester (Weeks 13–26)

Often called the “golden trimester,” many women feel their best during these weeks as nausea subsides and energy returns. The anatomy scan, typically performed around week 18 to 20, is a detailed ultrasound that checks the baby's organs, limbs, and overall development. This is often when parents learn the baby's sex. By week 24, the fetus reaches a critical milestone known as the threshold of viability — the point at which survival outside the womb becomes possible with intensive medical support, though outcomes at this early stage vary widely.

Third Trimester (Weeks 27–40)

During the final trimester, the baby gains significant weight, the lungs mature, and the brain undergoes rapid development. The baby typically settles into a head-down position in preparation for birth. Prenatal visits become more frequent, and providers begin monitoring for signs of preterm labor, preeclampsia, and other late-pregnancy complications.

Key Milestones by Week

What “Full Term” Really Means

The definition of “full term” pregnancy has evolved. For years, any birth between 37 and 42 weeks was broadly considered term. In 2013, the American College of Obstetricians and Gynecologists (ACOG) introduced more specific categories to reflect the fact that babies born at 37 or 38 weeks have higher rates of certain complications compared to those born at 39 or 40 weeks. The current designations are:

• 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

This distinction matters because even a week or two of additional development in the womb can improve outcomes related to brain development, lung function, and feeding ability. It is one reason why elective deliveries before 39 weeks are generally discouraged unless there is a medical indication.

Only About 5% of Babies Arrive on Their Due Date

Perhaps the most important thing to understand about your estimated due date is that it is exactly that — an estimate. Studies consistently show that only about 4 to 5 percent of babies are born on their exact due date. The majority arrive within a window of about two weeks on either side. A normal, healthy pregnancy can last anywhere from 37 to 42 weeks, and individual variation is completely natural.

Think of your due date as the middle of a range rather than a deadline. First-time mothers, on average, tend to deliver a few days past their due date. Setting expectations around a “due month” or “due window” rather than a single date can reduce anxiety in those final weeks of waiting.

Factors That Affect Due Date Accuracy

Several factors can make your LMP-based due date less reliable:

• Irregular cycles: Women with cycles shorter than 25 days or longer than 35 days may ovulate earlier or later than day 14, shifting the actual conception date.
• Recent use of hormonal birth control: Coming off the pill, patch, or hormonal IUD can delay the return of regular ovulation patterns.
• IVF and assisted reproduction: In IVF pregnancies, the due date is calculated from the date of embryo transfer rather than LMP, making it unusually precise.
• Uncertain LMP: If you are unsure of your last period date, first-trimester ultrasound becomes the primary dating method.
• Maternal factors: Age, BMI, ethnicity, and whether this is a first pregnancy can all subtly influence the length of gestation.

When to Talk to Your Doctor About Your Due Date

Your healthcare provider will typically confirm or adjust your due date at your first prenatal visit, usually between weeks 8 and 12. It is important to discuss your due date with your provider if:

• You have irregular menstrual cycles or are unsure of your LMP
• Your ultrasound measurements differ significantly from your LMP-based estimate
• You conceived through IVF or other assisted reproductive technologies
• You are carrying multiples (twins, triplets), which often deliver earlier
• You have a medical condition that may affect the timing of delivery

Once your due date is established (ideally by the end of the first trimester), it generally should not be changed based on later ultrasound measurements, as those are less accurate for dating purposes.

Calculate Your Due Date

Ready to estimate your own due date? Our free pregnancy calculator uses the same methods described above to give you an estimated due date, your current week and trimester, and a timeline of key milestones throughout your pregnancy.