Birthday Pregnancy Calculator
Conception Date From Birthday works out Probable date of conception and That was a from Date of birth, Gestational age at birth (weeks) and Gestational age at birth (extra days). The default figures on this page give probable date of conception of 2026-03-05; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
6 steps
- Gestational age at birth was 40w 0d = 280 days measured from the dating LMP.
- Conception is 14 days after that LMP, so it was 280 - 14 = 266 days before the birth.
- 2026-11-26 - 266 days = 2026-03-05.
- The fertile window is the six days ending on ovulation, so 2026-02-28 to 2026-03-05.
- Implantation follows 6-12 days later.
- Working forward again, conception + 266 days gives a due date of 2026-11-26.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the Conception Date From Birthday calculates
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed.
How to use this Conception Date From Birthday
Enter Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days) and Average cycle length. Probable date of conception updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Probable date of conception, That was a and Intercourse window opens.
The method follows ACOG Committee Opinion No. 700: Methods for Estimating the Due Date, Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle and Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy.
What you can compare
Change one input at a time to see how it affects Probable date of conception, That was a and Intercourse window opens. The inputs you can vary are Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days), Average cycle length and Luteal phase length.
Understanding Probable date of conception, That was a and Intercourse window opens
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.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to calculate birth date when pregnant?
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed. Open “How this is worked out” beside the result to see each step for your own figures.
When was my baby conceived based on a birthday calculator?
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed. Put your own figures in above and the working updates beside the result. For scale, the default figures on this page give 2026-03-05.
What day will I give birth predictor?
Works backwards from a due date, a birth date or a last menstrual period to the probable date of conception, the intercourse window that could have produced it and the implantation window that followed. On this page it is computed from Date of birth, Gestational age at birth (weeks) and Gestational age at birth (extra days) and shown as Probable date of conception, That was a and Intercourse window opens.
Can I calculate my pregnancy week-by-week based on my due date?
It needs Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days), Average cycle length and Luteal phase length. Nothing else is required, and the figures stay in your browser.
What information do I need for the Conception Date From Birthday?
You need Date of birth, Gestational age at birth (weeks), Gestational age at birth (extra days), Average cycle length and Luteal phase length. The page opens with default figures that give probable date of conception of 2026-03-05, so you can see the scale before entering your own.
What does this Conception Date From Birthday show?
It shows Probable date of conception, That was a, Intercourse window opens, Intercourse window closes, Intercourse window, Implantation window opens, Implantation window closes and Implied last menstrual period, with the working and any warnings for your entries.