Birthday Pregnancy Calculator

From the discharge summary. 40 weeks if the baby arrived on the due date.
Probable date of conception
2026-03-05
Conception Date Calculator
That was a
Thursday
Intercourse window opens
2026-02-28
Intercourse window closes
2026-03-05
Intercourse window
2026-02-28 to 2026-03-05
Implantation window opens
2026-03-11
Implantation window closes
2026-03-17
Implied last menstrual period
2026-02-19
Estimated due date
2026-11-26
Gestational age at birth
40 0/7 weeks
Worked back from
Actual birth date at 40w 0d
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

6 steps
  1. Gestational age at birth was 40w 0d = 280 days measured from the dating LMP.
  2. Conception is 14 days after that LMP, so it was 280 - 14 = 266 days before the birth.
  3. 2026-11-26 - 266 days = 2026-03-05.
  4. The fertile window is the six days ending on ovulation, so 2026-02-28 to 2026-03-05.
  5. Implantation follows 6-12 days later.
  6. Working forward again, conception + 266 days gives a due date of 2026-11-26.

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