A due date is simple arithmetic: the first day of your last period plus 280 days. What surprises most people is how loose that number really is. Only about 1 in 20 babies arrive on the day itself. The due date isn't a deadline, it's the midpoint of a four-week window, and understanding how it's built tells you how much to trust it.
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. That feels odd, since you weren't pregnant yet on day one, but the LMP is the one date most people can actually name. Ovulation, and therefore conception, typically happens about two weeks later.
The 280-day figure is Naegele's rule, named for the German obstetrician who popularized it in the early 1800s. It assumes a 28-day cycle with ovulation on day 14: 280 days from the LMP works out to 266 days from conception, or 40 weeks on the calendar. The shortcut version, if you'd rather not count days: take the first day of your last period, subtract three months, add seven days, and add a year.
Say your last period started on March 10. Add 280 days and you land on December 15 of the same year. That's the whole calculation for a textbook cycle. Along the way, weeks are counted from the same starting point, which is why you're called "8 weeks pregnant" when the embryo has existed for about six.
The pregnancy due date calculator does this in one step and also tells you how many weeks along you are today, your current trimester, and the estimated conception date.
Naegele's rule quietly assumes everyone ovulates on day 14, and plenty of people don't. If your cycles run 33 days, you likely ovulate around day 19, so a due date built on the 28-day assumption lands about five days too early. The fix is simple: adjust by the difference. A 33-day cycle adds 5 days to the standard date; a 25-day cycle subtracts 3.
If your cycles are irregular enough that no single length describes them, the LMP method loses its footing, and an early ultrasound becomes the better dating tool. Tracking ovulation directly helps too; an ovulation calculator can estimate your fertile window from your average cycle length.
Enter your last period and cycle length. The calculator adjusts the 280-day rule to your cycle and shows your week, trimester, and conception date.
Pregnancy Due Date Calculator →The most accurate dating comes from a first-trimester ultrasound, which measures the embryo from crown to rump. Early embryos grow at a remarkably uniform rate, so between about 8 and 13 weeks that measurement pins gestational age down to within 5 to 7 days. The later the scan, the more individual growth variation creeps in and the wider the error bars get.
That's why providers follow a specific rule for when an ultrasound overrides your period-based date. The American College of Obstetricians and Gynecologists (ACOG) re-dates the pregnancy when the scan and the LMP disagree by more than these margins:
| Ultrasound timing | Date changes if discrepancy exceeds |
|---|---|
| Up to 8 weeks 6 days | More than 5 days |
| 9 weeks to 15 weeks 6 days | More than 7 days |
| 16 weeks to 21 weeks 6 days | More than 10 days |
| 22 weeks to 27 weeks 6 days | More than 14 days |
| 28 weeks and later | More than 21 days |
Once a due date is set by an early scan, it stays set. A baby measuring two weeks ahead at 34 weeks is a big baby, not an older one.
Here's the honest answer the single printed date hides: about 4 to 5 percent of babies arrive on their due date, and roughly 80 percent arrive within the two weeks either side of it. First-time parents tend to deliver a bit later than the average. Doctors describe the finish line in bands rather than a single day:
| Category | Weeks of pregnancy |
|---|---|
| Preterm | Before 37 weeks |
| 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 |
| Postterm | 42 weeks and beyond |
So a due date of December 15 really means "very likely between December 1 and December 29." Plan the hospital bag for the start of that window, not the printed date. And if you're counting down, a general due date calculator works for project deadlines too, though babies ignore Gantt charts.
Your due date is the LMP plus 280 days, adjusted for your cycle length, and possibly corrected once by an early ultrasound. After that it's fixed, and it's best read as the center of a window: eight in ten babies arrive within two weeks of it, and only one in twenty on the day. Use it to pace your preparation, not to schedule your life to a Tuesday.
Yes, and it usually happens early. If a first-trimester ultrasound disagrees with your period-based date by more than about 5 to 7 days, your provider will move the date to match the scan. After that, the due date is normally locked in; a baby measuring big or small at 32 weeks is a growth finding, not a reason to re-date the pregnancy.
A first-trimester ultrasound that measures the embryo from head to bottom, called crown-rump length. Between roughly 8 and 13 weeks it predicts gestational age within about 5 to 7 days, because embryos grow at a very uniform rate early on. Accuracy drops as pregnancy progresses, which is why later scans rarely change the date.
From the transfer date, and it's the most precise dating there is because the embryo's exact age is known. For a day-5 embryo, add 261 days to the transfer date; for a day-3 embryo, add 263 days. No last-period guesswork and no cycle-length adjustment needed.
Only about 4 to 5 percent, roughly 1 in 20. The due date marks the middle of a window, not a deadline. Around 8 in 10 babies arrive in the two weeks either side of it, and first babies tend to run a little later than average.