Pregnancy Calculator · Practical guide
When Did I Conceive Based on My Due Date?
Work backward 266 days from your due date to find the likely day of conception.
At a glance
The Backward Formula
Conception typically happens about 266 days before the due date. The calculator works it out in two steps: your implied last menstrual period is the due date minus 280 days, and conception follows the LMP by about 14 days — so 280 minus 14 lands you at 266.
Interactive tool
Pregnancy Calculator
Enter your own values, then compare the result with the examples below.
Milestones
| Week | Milestone | Date |
|---|---|---|
| Week 8 | Heartbeat detectable | -- |
| Week 12 | End of first trimester | -- |
| Week 20 | Anatomy scan | -- |
| Week 27 | Start of third trimester | -- |
| Week 40 | Estimated due date | -- |
Worked Examples
| Due Date | Implied LMP | Estimated Conception |
|---|---|---|
| March 8, 2027 | June 1, 2026 | June 15, 2026 |
| December 25, 2026 | March 20, 2026 | April 3, 2026 |
Why It Is an Estimate
The 266-day figure assumes ovulation on day 14 of a 28-day cycle. Ovulation can shift by several days in either direction, and sperm can survive for up to about five days, so intercourse several days before ovulation can still lead to conception. A longer or shorter cycle moves the true conception date by the same number of days it moves the due date.
How Far Along That Makes You
Knowing the conception window also tells you your current week. In the first example, the LMP of June 1, 2026 means that on September 6, 2026 the pregnancy is at 13 weeks and 6 days — early in the second trimester, with the anatomy scan around week 20 (October 19, 2026) still ahead.
Frequently Asked Questions
Can conception happen days after intercourse?
Yes. Sperm can survive for up to about five days, so intercourse during the fertile window can result in fertilization when ovulation occurs days later.
Is the conception date the day a pregnancy test turns positive?
No. Implantation happens roughly 6 to 10 days after conception, and most tests only turn positive around the time of a missed period — about four weeks after the LMP.
What if I know my conception date but not my due date?
Add 266 days to the conception date to estimate the due date, then confirm with your provider or an early ultrasound.
Does an ultrasound change the conception estimate?
It can. If an early ultrasound re-dates the pregnancy, the implied conception date shifts by the same number of days as the revised due date.