To calculate your due date from conception, add 266 days to the date you conceived. That puts your estimated delivery date at 38 weeks after fertilization, which lines up with the standard 40-week pregnancy timeline counted from the last menstrual period (since ovulation typically happens about two weeks into a cycle). The math is straightforward if you know your conception date, but pinpointing that date is where things get tricky.
The 266-Day Formula
Human pregnancy lasts an average of 266 days from the moment of fertilization. To use this formula, take the date you believe conception occurred and count forward 266 days. That’s your estimated due date.
For a quick shortcut: go forward 8 months and 26 days from conception, or roughly 38 weeks. If you conceived on January 10, for example, your estimated due date would be October 3.
This is essentially the same result you’d get using the more common method doctors use, which starts from the first day of your last menstrual period and counts 280 days (40 weeks). The 14-day difference between the two methods accounts for the gap between the start of your period and ovulation. Both approaches land on the same date when the cycle is a textbook 28 days.
Why Your Conception Date May Not Be Exact
Most people estimate their conception date based on when they had sex, but fertilization doesn’t always happen that same day. Sperm can survive inside the reproductive tract for up to five days, while a released egg lives for less than 24 hours. The highest pregnancy rates occur when sperm and egg meet within four to six hours of ovulation. So if you had sex on a Monday but didn’t ovulate until Wednesday, conception likely happened on Wednesday, not Monday.
This is one reason doctors rarely use a conception-based timeline in clinical settings. Unless you were tracking ovulation closely, the actual fertilization date could be off by several days in either direction.
How to Narrow Down Your Ovulation Date
If you were tracking your cycle before getting pregnant, you may have a more precise conception date than most people. Basal body temperature charting is one common method: your resting temperature rises slightly (less than half a degree Fahrenheit) after ovulation and stays elevated for three or more days. Your most fertile window is the two days before that temperature shift, which is when conception most likely occurred.
Electronic fertility monitors that measure hormone levels in urine can also identify your fertile window. If you got a positive result on an ovulation predictor kit, ovulation typically happens within 24 to 36 hours of that surge. That gives you a reasonably tight window for estimating conception.
Without any tracking data, you can estimate ovulation as 14 days before your next expected period. For someone with a regular 28-day cycle whose last period started on March 1, that puts ovulation around March 15, making the estimated due date December 6 (266 days later). If your cycle is longer or shorter than 28 days, adjust the ovulation estimate accordingly. A 32-day cycle means ovulation likely happened around day 18, not day 14.
How IVF Changes the Calculation
If you conceived through IVF, you have the most precise conception date possible. The calculation starts from the embryo transfer date and subtracts the age of the embryo to find the equivalent conception date. For a day-5 embryo transfer, conception is considered to have occurred five days before the transfer. From there, add 266 days to get your due date.
For frozen embryo transfers at the blastocyst stage, whether the embryo was frozen on day 5, 6, or 7, clinics generally use the same due date calculation. The small difference in embryo age at freezing doesn’t meaningfully change the estimated delivery date.
First Pregnancies Often Run Longer
The 266-day figure is an average, and your actual delivery date can vary significantly. A study published in Obstetrics & Gynecology found that first-time mothers carried for a median of 274 days from ovulation, a full eight days longer than the standard 266-day prediction. Women who had given birth before delivered at a median of 269 days, still three days past the estimate.
This means if you’re pregnant for the first time, a more realistic due date might be 274 days after conception rather than 266. For a second or later pregnancy, 269 days may be closer to the mark. These are medians, though, not guarantees. Healthy pregnancies routinely end anywhere from 259 to 287 days after conception.
How Ultrasound Compares to Conception Dating
Even with a known conception date, your provider will likely confirm or adjust your due date with an early ultrasound. First-trimester ultrasounds (up to about 14 weeks) measure the embryo from head to rump and can estimate gestational age within a window of plus or minus five to seven days. That’s roughly the same margin of error as a well-tracked conception date.
In practice, ultrasound dating often wins out. One study found that 40% of women who received a first-trimester ultrasound had their due date adjusted because it differed from their period-based estimate by more than five days. The standard period-based method assumes a 28-day cycle with ovulation on day 14, which doesn’t account for irregular cycles, mistaken period dates, or later-than-expected ovulation. If your ultrasound date and your conception-based date are within a week of each other, they’re effectively in agreement.
If the two dates differ by more than a week, your provider will typically go with the ultrasound measurement, since it reflects actual embryo size rather than a calendar estimate. This doesn’t mean your conception date was wrong. It just means the baby’s growth pattern points to a slightly different timeline.
Quick Reference for Conception-Based Dating
- Standard formula: Conception date + 266 days = estimated due date
- First pregnancy adjustment: Conception date + 274 days may be more accurate
- Later pregnancies: Conception date + 269 days is a closer median
- IVF formula: Transfer date minus embryo age (in days) = conception date, then add 266 days
- Converting from LMP: Last menstrual period + 280 days gives the same result as conception + 266 days, assuming ovulation on cycle day 14
Keep in mind that only about 5% of babies arrive on their estimated due date. The calculation gives you a target to plan around, not a fixed appointment. A window of two weeks on either side of your due date is considered full term.