EDD stands for estimated due date, the projected date a pregnant person is expected to deliver their baby. It marks the completion of 40 weeks of gestation, counted from the first day of the last menstrual period. Only about 5% of babies are actually born on their EDD, but establishing this date early in pregnancy is one of the most important steps in prenatal care because nearly every decision about monitoring, testing, and intervention is built around it.
How EDD Is Calculated
The most common method for calculating an estimated due date is Naegele’s Rule, a simple three-step formula. According to Johns Hopkins Medicine, you take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. If your last period started on March 10, for example, you’d count back to December 10 and then add a year and seven days, giving you a due date of December 17.
Naegele’s Rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts accordingly. Someone with a 35-day cycle ovulates roughly a week later than expected, which would push the true due date about a week further out than the standard formula suggests. This is one reason providers don’t rely on the formula alone.
The Role of Ultrasound in Dating
Ultrasound measurements, particularly in the first trimester, are the most accurate way to confirm or adjust an EDD. During early pregnancy, a provider measures the embryo from head to rump. Because embryos grow at a remarkably predictable rate in the first 12 weeks or so, this measurement can pin down gestational age within just a few days. Later ultrasounds are less precise because babies start growing at different rates based on genetics, nutrition, and other factors.
The American College of Obstetricians and Gynecologists (ACOG) recommends that the EDD be established using the last menstrual period, the first accurate ultrasound, or both, and then documented clearly in the medical record. When there’s a meaningful discrepancy between the period-based calculation and the ultrasound measurement, providers will typically adjust the due date to match the ultrasound, especially if it was done early in pregnancy.
EDD for IVF Pregnancies
For pregnancies conceived through IVF or other assisted reproduction, the calculation is more straightforward because the exact date of conception is known. The due date is determined by taking the embryo transfer date, subtracting the age of the embryo at the time of transfer (three days for a cleavage-stage embryo, five or six days for a blastocyst), and then adding 266 days. This removes the guesswork about ovulation timing entirely, making the EDD for IVF pregnancies one of the most precise estimates possible.
Why EDD Matters for Your Care
An accurate EDD isn’t just about circling a date on the calendar. It serves as the reference point for virtually every aspect of prenatal care. Certain screening tests need to happen within specific gestational windows to be reliable. Growth checks compare your baby’s size against what’s expected for that point in pregnancy. If the due date is off by even a week or two, a baby growing normally could look too small or too large, potentially triggering unnecessary worry or interventions.
The EDD also determines how a pregnancy is classified as it progresses toward delivery. Pregnancies between 37 and 42 weeks are considered term, but the distinctions within that window matter. Babies born at 37 or 38 weeks (early term) face slightly different risks than those born at 39 or 40 weeks (full term). A pregnancy that reaches 42 weeks, or 14 days past the EDD, is classified as postterm. Postterm pregnancy carries increased risks for both the baby and the birthing parent, which is why providers monitor more closely as the due date approaches and passes.
How Accurate the Estimate Really Is
The word “estimated” in EDD is doing real work. About 5 in 100 people give birth on their actual due date. The rest deliver across a wide window, with anything between 37 and 42 weeks considered normal. Most first-time parents deliver a few days after their due date, while people who have given birth before tend to deliver slightly closer to or before it.
Think of the EDD less as a deadline and more as the center of a target. Your baby is most likely to arrive within a week or two on either side of that date. The precision of the estimate depends largely on when dating was done. A due date set by a first-trimester ultrasound is the gold standard. One based solely on a recalled last period, especially if your cycles are irregular, carries more uncertainty. Regardless of how it’s established, the EDD gives you and your care team a shared timeline for planning the weeks ahead.