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.
Methodology and sources
What this tool estimates
This calculator estimates when conception most likely occurred for a recognised pregnancy. It gives a single best estimate plus short windows for intercourse and conception. Results are approximate and based on typical cycle patterns.
Core pregnancy dating assumptions
The model follows standard obstetric dating. A typical pregnancy is treated as about 280 days from the first day of the last menstrual period and about 266 days from conception. The tool assumes that ovulation and conception happen around 14 days before the next period in an average cycle, while recognising that real cycles vary in length.
LMP and cycle length method
When you enter the first day of your last period and your usual cycle length, the calculator:
- treats the last period date as day 1 of the cycle,
- assumes ovulation happens about 14 days before the expected next period, and
- sets the likely conception date equal to this ovulation day.
The cycle length option lets the ovulation point move earlier or later for shorter or longer cycles, within a sensible range.
Due date method
When you enter an estimated due date, the calculator:
- counts back 266 days from the due date to estimate the day of conception, and
- treats this as the ovulation day for the intercourse and conception windows.
This matches the common clinical rule that adds 280 days to the first day of the last period for a 28 day cycle.
Ultrasound method
When you enter an ultrasound date and gestational age in weeks and days, the calculator:
- uses the weeks and days to estimate how many days have passed since the start of the pregnancy dating point,
- counts back that number of days from the scan date to estimate an equivalent last period date, then
- adds 14 days to approximate the ovulation and conception day.
This reflects common use of early ultrasound to refine pregnancy dating when the last period is uncertain.
Intercourse and conception windows
The tool uses simple rules around the estimated ovulation day:
- Most likely intercourse day is set to the day before ovulation.
- The intercourse window runs from 5 days before ovulation up to the ovulation day, reflecting sperm survival in cervical and uterine mucus.
- The conception window runs from the ovulation day through the following day, reflecting the brief time the egg is available.
These windows are designed for easy counselling. They do not capture all possible variation in ovulation timing or sperm and egg survival.
Corresponding due date
For consistency, the calculator also reports a due date that matches the estimated conception day by adding 266 days. This may differ slightly from a due date set by your clinic if different dating rules or measurements were used.
Limits of the estimate
Ovulation and conception cannot be known exactly from calendar dates alone. Cycle length can vary from month to month, and ultrasound measurements have a margin of error. This tool does not use hormone tests, ovulation predictor kits, or detailed ultrasound measurements, and it is not designed for legal or paternity decisions.
Disclaimer
This tool is for general education only and is not medical advice. It does not replace personalised assessment and counselling with your maternity care team. Always discuss your own results and questions with your clinician.
Sources
Guidance and data informing this tool include:
- Wilcox AJ, Weinberg CR, Baird DD. "Timing of sexual intercourse in relation to ovulation." New England Journal of Medicine. 1995 (data on ovulation, fertile window, and dating).
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 is not always instant and the egg is viable for up to 24 hours, we then add two days to estimate a likely conception date.
From Due Date: This is a 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.
From an Ultrasound: This is often the most reliable method, especially if done in the first trimester. An ultrasound gives a snapshot of your baby's age on a specific day. We use that data to estimate 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.
Medical disclaimer
The information and calculators on this site are educational tools only. They provide statistical estimates based on published research and the details you enter.
They cannot diagnose, predict what will happen for you, or replace personalized advice from a licensed health care professional who knows your full history. Always talk with your doctor, midwife, or other qualified clinician before making decisions about your health, fertility, or pregnancy.
Never ignore, avoid, or delay seeking professional medical advice because of something you read here or a result you see in a calculator. If you think you may be having a medical emergency, call 911 in the United States or your local emergency number.
- Wilcox AJ, Dunson D, Weinberg CR, et al. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine. 1995;333:1517-1521. Classic study on ovulation timing and the fertile window. View article.
- Morgan JA, Cooper DB. Pregnancy dating. StatPearls [Internet]. Treasure Island, FL: StatPearls Publishing. Summary of clinical methods for pregnancy dating and due date estimation. View chapter.