To estimate when you conceived, subtract 266 days (38 weeks) from your due date. That gives the approximate day of ovulation and fertilization if your due date is accurate and you ovulated around day 14 of your cycle. Because sperm can survive for several days and ovulation timing varies, the sex that led to pregnancy most likely happened in the five or so days before that date, or on the day itself. If your due date came from an early ultrasound rather than your last period, it already accounts for when you actually ovulated, which makes the estimate more reliable.
"When did I conceive?" can be curiosity, a wish to remember a moment, or a question with real stakes. Whatever your reason, the honest answer is a short window of days, not a single date. Understanding why starts with how due dates are calculated in the first place.
How Your Due Date Was Set
From your last menstrual period (LMP)
By convention, your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, according to the American College of Obstetricians and Gynecologists (ACOG). This method assumes a 28-day cycle with ovulation around day 14. So "40 weeks pregnant" includes about two weeks before you conceived. Not sure which day counts as the first day of your period? FertileStart explains what counts as cycle day 1.
From an early ultrasound
ACOG says that measuring the embryo or fetus by ultrasound in the first trimester (up to 13 weeks and 6 days) is the most accurate way to establish or confirm gestational age. If the ultrasound and your LMP disagree by enough, your doctor changes the due date. ACOG's thresholds: before 9 weeks, a difference of more than 5 days; from 9 weeks through 13 weeks and 6 days, a difference of more than 7 days.
From IVF
If you conceived through IVF, ACOG says the due date should be based on the age of the embryo and the date of transfer. In that case, you already know when fertilization happened to within a day.
Use the due date your doctor or midwife is using now. If an early ultrasound changed it, the new date is the better starting point, because it reflects how far along the pregnancy actually measured.
The Math: Counting Back
Three quick calculations
A worked example
Say your due date is June 20, 2027.
- Subtract 280 days: the estimated first day of your last period is September 13, 2026.
- Subtract 266 days: estimated ovulation and conception is about September 27, 2026.
- Likely window for the sex that led to pregnancy: roughly September 22 to September 27, 2026, with the days closest to ovulation being the most likely.
Now suppose your cycles usually run about 35 days and you ovulated around day 21 instead of day 14. Then conception would have been closer to October 4, and an early ultrasound would likely have measured the pregnancy as about a week behind your LMP dates, moving your due date later. This is exactly why ultrasound dating matters for people with long or irregular cycles. FertileStart's guide to late ovulation causes explains why ovulation can shift.
Use Our Calculators to Check Your Window
Our tools work forward from a date, so you can test a candidate date against your due date:
| Tool | What to enter | What it tells you |
|---|---|---|
| Due date calculator | Use the "Conception Date" tab and enter a date you think you conceived | The due date that date would produce (it adds 266 days). If it matches your official due date, that date is consistent. |
| Implantation calculator | Enter an ovulation date, your last period, or an IVF transfer date | When implantation likely happened, which helps make sense of early spotting or symptoms |
| Pregnancy test calculator | Enter your last period or ovulation date | When a test would first have been reliable, a useful cross-check with when you tested positive |
If you remember a positive test date, that is a helpful clue: implantation, and then a positive test, come some days after conception. Our explainers on implantation and the full ovulation-to-delivery timeline lay out the sequence.
Why You Cannot Pin Down a Single Day
- The fertile window is several days long. ASRM's patient fact sheet describes it as about six days, ending on the day of ovulation. Sex on any of those days could have resulted in pregnancy. Our guide to the fertile window explains why.
- Ovulation varies. Even people with regular cycles do not always ovulate on the same day, and stress, illness or travel can shift it.
- Dating has a margin of error. Even early ultrasound dating is an estimate, which is why ACOG allows several days of difference before changing a due date.
- Memory is imperfect. Many people are not sure of their exact LMP date.
Situations That Make Counting Back Harder
- You conceived before a period returned. If you got pregnant while breastfeeding or soon after stopping hormonal birth control, you may not have a recent period to count from. Ultrasound dating is especially important here.
- You had bleeding you thought was a period. Light bleeding in early pregnancy is sometimes mistaken for a period, which can make LMP dating off by weeks. An early ultrasound usually catches this.
- Your cycles are irregular. With irregular cycles, ovulation could have happened on many different days, so LMP dating is rough. Again, the ultrasound-based due date is the one to trust.
- Your first ultrasound was later in pregnancy. Dating becomes less precise as pregnancy goes on, so a due date set in the second trimester gives a wider window.
When the Exact Date Really Matters
Sometimes people ask because they need to know who the biological father is. Counting back from a due date cannot answer that question reliably, especially if intercourse with different partners happened within about a week of each other. Genetic (DNA) paternity testing, available during pregnancy or after birth, is the reliable way to know. A doctor or a reputable accredited lab can explain the options.
If you need a date for another reason, such as medical records or a medication exposure question, tell your doctor what you know. They can work with the ultrasound measurements in your chart.
For a forward-calculating tool and more on due-date math, FertileStart's due date calculator guide is a good companion.
Frequently Asked Questions
The Bottom Line
You can estimate when you conceived by counting back 266 days from your due date, then allowing for a window of several days before that. If an early ultrasound set your due date, the estimate is more reliable; if you have long or irregular cycles and only LMP dating, it is rougher. Use our due date calculator to test candidate dates, and remember that for paternity questions, only DNA testing gives a firm answer.
Check a date against your due date
Enter a possible conception date and see which due date it produces.
Open the Due Date Calculator →- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017). https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2017/05/methods-for-estimating-the-due-date
- ASRM ReproductiveFacts, Optimizing Natural Fertility patient fact sheet. https://reproductivefacts.org/globalassets/_rf/news-and-publications/bookletsfact-sheets/english-pdf/optimizing_natural_fertility-factsheet.pdf