EDD stands for estimated date of delivery, sometimes called the estimated due date. It’s the single calendar date that marks 40 weeks (280 days) from the first day of your last menstrual period, giving you and your care team a target for when your baby is most likely to arrive. Only about 5% of babies are born on that exact date, but it serves as the anchor point for every prenatal milestone, test, and decision throughout pregnancy.
How Your Due Date Is Calculated
The most common method is called Naegele’s rule, and it’s simple enough to do on paper. 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 June 10, 2024, you’d count back to March 10, then add a year and seven days to land on March 17, 2025.
This formula assumes a 28-day menstrual cycle with ovulation happening right around day 14. That’s a fine average, but plenty of people have cycles that run shorter or longer. If your cycle is consistently 35 days, for example, you likely ovulated a week later than the formula assumes, which means your real due date could be about a week later too. That built-in limitation is one reason providers often confirm or adjust the date with an ultrasound.
Why Ultrasound Dating Is More Accurate
A first-trimester ultrasound, done before 14 weeks, is the most accurate way to establish a due date. The technician measures the baby from the top of the head to the bottom of the torso (a measurement called crown-rump length), and that number corresponds to a specific gestational age. Earlier in the first trimester, the measurement is even more precise because embryos grow at a remarkably uniform rate in those early weeks, regardless of genetics or other factors.
The margin of error for a first-trimester ultrasound is plus or minus 5 to 7 days. That’s considerably tighter than what you get from period-based math alone, especially if your cycles are irregular or you aren’t sure of the exact start date. If the ultrasound date and the period-based date are close, your provider will generally keep the original. If they differ by more than a certain threshold, the ultrasound date replaces it.
Ultrasounds done later in pregnancy are less reliable for dating because babies start growing at different rates. A second-trimester scan still provides useful information, but the window of error widens, and a third-trimester scan is too imprecise to change a due date at all.
Due Dates for IVF Pregnancies
If you conceived through in vitro fertilization, your due date is calculated differently because the exact dates of egg retrieval and embryo transfer are known. There’s no guesswork about when ovulation happened. The calculation is typically about two weeks shorter than the standard method because there’s no need to estimate the gap between your period and conception. For frozen embryo transfers at the blastocyst stage, the due date is the same whether the embryo was frozen on day 5, day 6, or day 7, since all are at the same developmental stage when transferred.
What “Full Term” Actually Means
Your EDD falls at 40 weeks, but the reality is that healthy babies arrive across a range of weeks. The American College of Obstetricians and Gynecologists breaks it down into four categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Postterm: 42 weeks and beyond
Most babies arrive between 37 and 41 weeks, typically within a week of the due date in either direction. The “full term” window doesn’t even begin until 39 weeks, which is why providers generally discourage elective early delivery. Those last few weeks matter for lung development, brain growth, and the baby’s ability to regulate temperature and feeding after birth.
Why the Date Matters Even If It’s Imprecise
It might seem odd that so much of prenatal care revolves around a date that only 5% of babies actually hit. But the EDD isn’t really a prediction of when you’ll deliver. It’s a reference point that your entire pregnancy timeline hangs on. Routine blood tests, genetic screenings, and growth checks are all scheduled relative to your gestational age, which is counted forward from that due date anchor.
The date also determines how your provider responds if labor starts early or hasn’t started late. A baby arriving at 34 weeks needs different preparation than one arriving at 39. And if you reach 41 or 42 weeks, the conversation shifts toward whether to induce labor, since risks to the placenta and baby increase gradually past that point. Having an accurate EDD makes those decisions more precise, which is why getting the dating right early on carries real consequences later.
If your due date gets adjusted after an ultrasound, that’s normal and not a sign that anything is wrong. It simply means the measurement gave a clearer picture of how far along you are than your period date alone could provide.