How Is Your Due Date Calculated?
The moment most people see a positive pregnancy test, the next question is "okay, so when am I actually due?" It feels like it should be simple — count forward nine months — but the real formula is a little different, and understanding it makes the whole rest of pregnancy easier to follow. It also explains why your due date might shift slightly after your first ultrasound, which can be unsettling if you don't know it's completely normal.
Here's the short version: your due date isn't counted from conception. It's counted from the first day of your last menstrual period (LMP) — even though you weren't actually pregnant yet on that day. That one quirk explains most of the confusion around due date math.
The standard formula: Naegele's Rule
The method used by healthcare providers worldwide is called Naegele's Rule, named after the 19th-century obstetrician who popularized it. It's straightforward: take the first day of your last menstrual period and add 280 days — exactly 40 weeks.
Forty weeks is the standard length used to describe a full-term pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG) and the CDC. It isn't a hard deadline — it's an average, a midpoint around which actual delivery dates are spread. A healthy pregnancy can end anywhere from 37 to 42 weeks, and in fact only a small share of babies — often cited as around 5% — are actually born on their exact due date. Most arrive within a week or two of it, in either direction.
Our Due Date Calculator runs this same math for you automatically. Enter the first day of your last period, and it adds 280 days and shows you the result along with your current week of pregnancy and trimester.
Why it's dated from your last period, not conception
This trips a lot of people up. You weren't pregnant on the first day of your last period — you likely conceived around two weeks later, near ovulation. So why start counting there?
It's a matter of practicality. Most people know roughly when their last period started, but far fewer know the exact day they conceived, since that depends on the day sperm met egg, not the day of intercourse. Using the LMP as a fixed, rememberable starting point made dating consistent across patients long before ovulation tracking or early ultrasounds existed, and the convention has stuck because it works well on average.
The practical effect is that when you're told you're "6 weeks pregnant," you've actually only been pregnant for about 4 weeks since conception. The extra two weeks account for the time between your last period and ovulation.
What if your cycle isn't 28 days?
Naegele's Rule assumes a textbook 28-day cycle with ovulation around day 14. If your cycles typically run longer or shorter than that, a due date based purely on LMP without adjustment can be off by several days.
For example, someone with a consistent 35-day cycle likely ovulates later in their cycle than someone with a 24-day cycle — which shifts the realistic conception date, and therefore the due date, later as well. A good due date calculator accounts for this by asking for your average cycle length and adjusting the estimate accordingly, rather than assuming everyone ovulates on day 14.
If you know your conception date more precisely — say, from fertility tracking or a known ovulation date — some calculators, including ours, let you calculate from that date instead of the LMP, which can be more accurate if your cycles are irregular.
How ultrasound dating fits in
During the first trimester, ultrasound measurements — particularly crown-rump length, the measurement from the top of the head to the bottom of the torso — give a very reliable estimate of gestational age. Because early fetal growth happens at a fairly predictable rate, an ultrasound done between roughly 8 and 13 weeks is generally considered the most accurate way to confirm or adjust a due date, according to ACOG guidance.
If your LMP-based date and your ultrasound-based date are close together, your provider will usually keep the original due date. If they differ more than a small, clinically meaningful margin, your provider may revise your due date to match the ultrasound estimate instead — especially if your periods are irregular or you're unsure of your LMP. That revision isn't a sign anything is wrong; it simply means the ultrasound gave a more precise estimate than the calendar math could.
We cover this in more detail in our guide on why and when due dates change.
The bottom line
Your due date is a well-reasoned estimate built from a simple, consistent formula — not a precise prediction of the day your baby will arrive. It's calculated by adding 40 weeks to the first day of your last period, adjusted for your typical cycle length, and refined by ultrasound measurements early in pregnancy. Knowing how the number is built makes it much easier to treat it as the useful guideline it's meant to be, rather than a deadline to watch anxiously.