Conception Date Calculator: From LMP, Due Date, or Ultrasound

Trying to pinpoint the exact start of your pregnancy journey? Our calculator makes it easy, allowing you to find your conception date from the information you have available—whether it’s your last period, an ultrasound scan, or your due date.

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.

  1. Choose Your Method: First, select the tab that matches the information you have: Last Period Date, Due Date, or Ultrasound.
  2. 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.
  3. Calculate Your Date: Click the “Calculate Conception Date >” button to see your estimated timeline.
A screenshot of the multi-tab Conception Date Calculator, showing the different input fields for Last Period Date, Due Date, and Ultrasound.

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.

A timeline that visually explains the fixed relationship between the three key dates: LMP, Conception, and Due Date. It shows how knowing any one point on the timeline allows you to "triangulate" the others.
¹All calculation methods are based on clinical standards for gestational timelines recognized by organizations like The American College of Obstetricians and Gynecologists (ACOG).

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 provides the most accurate data for determining gestational age and, therefore, the most reliable conception date estimate. If you have ultrasound data, use that tab for the best result.

Why does my cycle length matter for the LMP calculation?
The “Day 14” ovulation rule is based on a 28-day cycle. If your cycle is 32 days, you likely ovulate later (around day 18). If it’s 24 days, you likely ovulate earlier (around day 10). Adjusting for cycle length gives a much more personalized and accurate conception date estimate.

Can this calculator tell me the exact day I had intercourse?
No, and no calculator can. Conception is the moment of fertilization, but sperm can survive in the female reproductive tract for up to 5 days. This means intercourse could have occurred up to five days before the estimated conception date.

What if I have very irregular cycles?
If your cycles are highly irregular, the “Last Period Date” method will be the least accurate. Your best options are to use the “Ultrasound” method if you’ve had a scan, or the “Due Date” method if your doctor has already given you one (as they likely based it on an ultrasound).

Is the conception date the same as the implantation date?
No. Conception is when the egg is fertilized. Implantation is a later event, occurring 6 to 12 days after conception, when the embryo attaches to the uterine wall. You can estimate this date with our 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.