Pregnancy Conception Calculator
Find the conception date, last period and due date from a due date or conception date. Reverse-calculate your pregnancy timeline.
Enter a due date or conception date to see the related pregnancy dates.
Last updated: February 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Use the conception calculator when you want to work out the likely date a pregnancy began — whether you know the due date and want to trace it backwards, or you know the conception date and want to see the resulting LMP and due date. It helps answer the common question 'when did I conceive?' for records, for telling the family, or simply to understand how pregnancy dating works. Doctors, hospital forms and insurance paperwork in India routinely ask for the LMP and expected delivery date, so this tool helps you prepare those details before appointments. It also makes a useful cross-check: if your calculated dates and your scan dates disagree, you have a concrete question to raise with your doctor rather than a vague doubt.
How to Use This Calculator
- Step 1: Choose whether you know your due date or your conception date.
- Step 2: Pick the date you know on the calendar.
- Step 3: Read the calculated conception date, LMP and due date for the pair.
- Step 4: If you entered a due date, check the current gestational age too, then confirm with an ultrasound.
Worked Example
A couple in Chennai knows their expected due date is 5 October 2026 and wants to work backwards. The calculator subtracts 266 days and estimates conception on 12 January 2026, with the last period around 29 December 2025. On 14 August the pregnancy would be about 32 weeks and 4 days along. Working forward from a conception date of 15 January 2026 instead gives an LMP of 1 January and a due date of 8 October 2026. Both results follow the fixed 266-day and 280-day conventions used to date pregnancies.
Tips and Common Mistakes
- •Tip 1: If your cycle is regular, the LMP shown can help you cross-check which month you conceived.
- •Tip 2: An early ultrasound date is more trustworthy than a calculation, so update your plan after the first scan.
- •Tip 3: Remember that sex up to five days before ovulation can still cause conception, so the day shown is an estimate, not a proof.
- ✗Mistake 1: Assuming the conception date is exact — with sperm survival, the real day can differ by several days.
- ✗Mistake 2: Entering a rough or guessed due date and then relying on the gestational age shown, since bad input flows through every result.
Frequently Asked Questions
How is the conception date calculated from the due date?
The calculator subtracts 266 days (38 weeks) from the due date, because that is the average length of a pregnancy counted from conception rather than from the LMP.
Why is my LMP shown as 14 days before conception?
In a typical 28-day cycle, ovulation and the most fertile days fall around day 14. Dating backwards, the LMP is therefore about two weeks before the estimated conception date.
Can I be sure the conception date is exact?
No. The estimate is reliable to within a few days only if your cycle is regular and you know the due date was confirmed early. Sperm can survive up to five days, so sex in the days before ovulation can also lead to conception.
Why would the doctor give me a different due date?
Doctors often set the date from an early ultrasound scan, which measures the baby rather than assuming a 28-day cycle. If your scan date differs, the ultrasound date is usually considered more accurate.
Does this help me plan or avoid pregnancy?
It shows the window around ovulation when conception is most likely, which supports family planning conversations. It is not a contraceptive method — for reliable prevention, use a proper contraceptive.
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