Enter LMP, due date, or ultrasound to estimate your conception date and likely window. We also show a timeline of key pregnancy dates.
Conception Date Calculator
Find your estimated conception date using your last period, due date, or ultrasound scan.
How to Use This Calculator
This tool gives you three ways to find your estimated conception date.
- Choose Your Method: First, select the tab that matches the information you have: Last Period Date, Due Date, or Ultrasound.
- Enter Your Details:
- For Last Period Date: Enter the first day of your last menstrual period and your average cycle length.
- For Due Date: Enter the estimated due date given to you by your doctor.
- For Ultrasound: Enter the date of the scan and the gestational age (in weeks and days) determined at that time.
- Calculate Your Date: Click the “Calculate Conception Date >” button to see your estimated timeline.
The Science: Three Paths to the Same Date
While it feels like there should be one simple answer, your conception date can be estimated in several ways. The accuracy depends on the quality of the data you start with. As the timeline below illustrates, all methods work by navigating the fixed relationship between your Last Menstrual Period (LMP), your conception date, and your due date.
From Last Menstrual Period (LMP): This is the most common method. We start with your LMP and adjust for your unique cycle length to pinpoint your most likely ovulation date. Because fertilization isn’t always instant and the egg is viable for up to 24 hours, we then add two days to find the most accurate conception date.
From Due Date: This is a precise reverse calculation. We start at your due date and work backward 266 days (38 weeks) to find your estimated ovulation date. We then add two days to pinpoint the most likely date of conception, accounting for the time needed for fertilization. This is a more nuanced approach than many online tools.
From an Ultrasound: This is the gold standard for accuracy, especially if done in the first trimester. An ultrasound gives us a precise “snapshot” of your baby’s age on a specific day. We use that data to accurately determine your ovulation date, and from there, we add two days to find the most likely conception date.
Frequently Asked Questions
Which calculation method is the most accurate?
An early first-trimester ultrasound is most reliable for gestational age and, in turn, your conception-date estimate. If you have scan data, use the Ultrasound tab or see the Ultrasound Due Date Calculator.
Why does my cycle length matter for the LMP calculation?
Cycle length shifts typical ovulation timing. A 32-day cycle often ovulates near day ~18; a 24-day cycle near day ~10. Adjusting for your cycle makes the LMP-based estimate more accurate.
Can this calculator tell me the exact day I had intercourse?
No. Sperm can survive up to 5 days, so intercourse may have occurred several days before the estimated conception date.
What if I have very irregular cycles?
With irregular cycles, the Last Period method is least accurate. Use the Ultrasound method if you’ve had a scan, or the Due Date method if you already have an EDD: try the Pregnancy Due Date Calculator.
Is the conception date the same as the implantation date?
No. Conception is fertilization; implantation usually occurs about 6–12 days later. You can estimate it with the Implantation Calculator.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with a healthcare provider for any health concerns or to confirm your pregnancy timeline.
References
¹ The American College of Obstetricians and Gynecologists. (2017). Methods for Estimating the Due Date. Committee Opinion No. 700.