The most common way to estimate a due date is to count 280 days (40 weeks) from the first day of your last menstrual period. That puts the average pregnancy at about 40 weeks from that starting point, though the actual time from conception to birth is closer to 266 days, or roughly 38 weeks. Several methods exist for pinpointing this date, and each one works best in different circumstances.
The Last Menstrual Period Method
The classic formula, known as Naegele’s rule, uses three steps: take the first day of your last period, count back three calendar months, then add seven days and one year. If your last period started on March 10, for example, you’d count back to December 10, then add seven days and one year to get December 17. This formula assumes a 28-day cycle with ovulation happening on day 14.
The catch is that not everyone has a 28-day cycle. If your cycles are consistently longer, say 35 days, ovulation likely happens later than day 14, which means the standard formula would set your due date too early. The reverse is true for shorter cycles. If you know your typical cycle length, you can adjust by adding or subtracting the difference from 28 days. A 35-day cycle, for instance, means adding 7 extra days to the result you get from Naegele’s rule.
Using a Known Ovulation or Conception Date
If you’ve been tracking ovulation with test kits, basal body temperature, or other methods, you can skip the period-based math entirely. Count 266 days (38 weeks) forward from the date of conception or ovulation. This tends to be more precise than the last-period method because it removes the guesswork about when you actually ovulated. A large study published in Human Reproduction found that the median time from ovulation to birth was 268 days, with a standard deviation of about 10 days, meaning most deliveries clustered within a roughly three-week window around that median.
First-Trimester Ultrasound
An ultrasound performed in the first trimester (up to about 14 weeks) is the single most accurate method for establishing a due date. During this scan, the technician measures the length of the embryo or fetus from head to rump. That measurement, called the crown-rump length, can estimate gestational age to within 5 to 7 days. The earlier in the first trimester the scan is done, the more accurate it is.
Once the fetus grows past about 84 millimeters (around 14 weeks), this particular measurement becomes less reliable and other size markers are used instead. Those later measurements carry a wider margin of error, which is one reason early prenatal care matters for accurate dating.
When Ultrasound Overrides Your Period Date
It’s common for the ultrasound date and the period-based date to disagree. In one study, 40% of women who received a first-trimester ultrasound had their due date adjusted because the two methods differed by more than five days. When this happens, the ultrasound date generally takes priority because it’s based on direct measurement rather than assumptions about cycle length and ovulation timing. Once a due date is established through early ultrasound, it rarely gets changed again later in pregnancy.
IVF and Assisted Reproduction
If you conceived through IVF, the calculation is more straightforward because the exact date of embryo transfer is known. The formula works backward from the transfer date: subtract the age of the embryo at transfer (3 days for a cleavage-stage embryo, 5 or 6 days for a blastocyst) to find the conception date, then add 266 days. So if you had a day-5 blastocyst transferred on June 15, the estimated conception date would be June 10, and the due date would be 266 days from there. Guidelines from ACOG specify that this method should be used over ultrasound dating when assisted reproduction is involved.
Fundal Height as a Rough Check
Later in pregnancy, your provider may measure the distance from your pubic bone to the top of your uterus. After 24 weeks, this measurement in centimeters roughly matches the number of weeks pregnant you are, give or take about 3 centimeters. At 30 weeks, for example, the measurement would typically be around 27 to 33 centimeters. This isn’t used to set a due date, but it serves as a quick check that fetal growth is tracking as expected. Factors like body weight and uterine fibroids can throw off the measurement, and it becomes less reliable after 36 weeks.
How Precise a Due Date Really Is
Only about 5% of babies arrive on their exact due date. Most are born somewhere between 37 and 41 weeks, typically within a week of the predicted date. That study in Human Reproduction found that even when ovulation was precisely known, the natural variation in pregnancy length spanned a range of more than 5 weeks across all deliveries. The standard deviation was 10 days from the mean of 267 days post-ovulation.
This natural variability means your due date is best understood as the center of a window, not a deadline. Several biological factors influence when labor begins, and no formula can account for all of them. The practical takeaway: use the most accurate method available to you (early ultrasound if possible, known ovulation date if you have it, last period as a fallback), and treat the result as a strong estimate rather than a fixed appointment.