Due Conception Date Calculator

Probable date of conception
2026-01-15
Conception Date Calculator
That was a
Thursday
Intercourse window opens
2026-01-10
Intercourse window closes
2026-01-15
Intercourse window
2026-01-10 to 2026-01-15
Implantation window opens
2026-01-21
Implantation window closes
2026-01-27
Implied last menstrual period
2026-01-01
Estimated due date
2026-10-08
Worked back from
Estimated due date
Worked out from ACOG Committee Opinion No. 700: Methods for Estimating the Due Date and 2 other sources.
Ovulation varies between cycles even in regular cycles, so treat the single date as the centre of a window rather than a fact. Intercourse up to five days before ovulation can produce the pregnancy.
An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.

How this is worked out

5 steps
  1. The estimated due date is 40 0/7 weeks, and conception sits 14 days after the dating LMP, so it is 266 days before the due date.
  2. 2026-10-08 - 266 days = 2026-01-15.
  3. The fertile window is the six days ending on ovulation, so 2026-01-10 to 2026-01-15.
  4. Implantation follows 6-12 days later.
  5. Working forward again, conception + 266 days gives a due date of 2026-10-08.

Results are general information, not financial, tax, legal, medical or engineering advice.