From your due date
The due date convention is 280 days from the first day of the last period,1 and the convention assumes ovulation on day 14.1 280 minus 14 gives the 266-day interval used here.
Due date − 266 days
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Conception Date Calculator
When did I conceive? Get an estimated conception date from your due date or the first day of your last period. It follows the standard pregnancy-dating convention, so it’s an estimate rather than an exact date.
- Calculated on your device
- Sources listed
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Estimate a conception date
Choose the date you know. You can switch at any time.
Estimated conception date
Estimate · by convention
Based on:
An estimate, not an exact dateThe standard dating convention assumes ovulation on day 14 of a regular 28-day cycle and a due date 280 days after the first day of the last period.1 This calculator uses that convention to place an estimated conception date 266 days before the estimated due date. It is not an exact biological conception date — the calculator can’t determine that.
This estimate is for information only. Your midwife or doctor can talk you through your pregnancy dating.
Calculated at mypregnancyduedate.com/conception-date-calculator
How it works
How does a conception date calculator work?
It works backwards from pregnancy dating. Pregnancy is dated from the first day of the last period, not from conception.2 By convention, the estimated due date is 280 days after the first day of the last period,1 and the convention assumes ovulation on day 14 of a regular 28-day cycle.1 The calculator uses these conventions to estimate when conception is assumed to have happened.
Need a due date first? Use the Due Date Calculator. To see how far along you are today, use the Pregnancy Weeks Calculator.
How is the estimated conception date calculated?
Both methods use the same convention, so for the same pregnancy they give the same estimated date.
From your last period
The convention assumes ovulation on day 14 of a regular 28-day cycle,1 so the calculator adds 14 days to the first day of the last period.
LMP + 14 days
Is the conception date exact?
No. It is an estimate based on the dating convention. The convention assumes a regular 28-day cycle with ovulation on day 14, and it doesn’t account for irregular cycles, inaccurate recall of the last period, or variation in when ovulation happens.1 The calculator can’t determine the exact date conception happened.
Can an ultrasound tell the exact conception date?
Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.1 That result is given as gestational age, which is counted from the first day of the last period rather than from conception.2 The sources this site relies on describe ultrasound as a way to date a pregnancy, not as a way to identify an exact conception date. If your due date was set or changed after a scan, enter that due date here for a conventional estimate that matches it.
Why can the estimate differ from when conception happened?
FAQ
Conception date questions
Short answers, with sources. Your midwife or doctor can tell you more about your own dates.
How does a conception date calculator work?
It applies the standard pregnancy-dating convention in reverse. By convention the due date is 280 days after the first day of the last period, with ovulation assumed on day 14,1 so the calculator places an estimated conception date 266 days before the due date.
How is an estimated conception date calculated from a due date?
The calculator subtracts 266 days from the estimated due date. That interval comes from the 280-day due date convention minus the 14 days the convention assumes before ovulation.1
Is the estimated conception date exact?
No. The convention assumes a regular 28-day cycle and doesn’t account for irregular cycles or variable ovulation timing,1 so the result is an estimate. The calculator can’t determine the exact date conception happened.
Can I calculate the conception date from my last period?
Yes. Choose “Last period” above. The convention assumes ovulation on day 14 of a regular 28-day cycle,1 so the calculator adds 14 days. If your cycles are irregular, keep in mind that the convention doesn’t account for that.
Can an ultrasound determine the exact conception date?
Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is,1 and that is expressed as gestational age from the first day of the last period.2 The sources this site relies on don’t describe it as identifying an exact conception date.
Can this calculator determine paternity?
No. It gives an estimated date based on a dating convention. It does not establish paternity and should not be used to determine paternity.
Sources
- Methods for Estimating the Due Date. Committee Opinion No. 700. American College of Obstetricians and Gynecologists (ACOG), Committee on Obstetric Practice; American Institute of Ultrasound in Medicine; Society for Maternal-Fetal Medicine. Obstet Gynecol 2017;129:e150-4. Reaffirmed 2025. acog.org
- International Statistical Classification of Diseases and Related Health Problems, 10th Revision. Volume 2: Instruction manual. Sixth Edition, 2019. World Health Organization (WHO). 2019. icd.who.int
- Length of human pregnancy and contributors to its natural variation. Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Hum Reprod. 2013 Oct;28(10):2848-55. doi:10.1093/humrep/det297
Sources last checked 3 October 2026