Conception Date Calculator
Estimate when conception is likely to have happened — from the last period, from a due date, or from how many weeks along a scan says you are.
Pregnancy is dated from the first day of the last menstrual period, not from conception, because that date is the one people can usually remember.
How the conception date calculator works
Pregnancy is dated from the first day of the last menstrual period, not from conception, because that date is the one people can usually remember. Ovulation, and so conception, follows roughly 14 days later in a 28-day cycle — adjusted here for a longer or shorter cycle, since the second half of the cycle stays near 14 days while the first half varies. The fertile window spans about five days before ovulation to one day after, because sperm survive several days while an egg lives about 24 hours. A dating scan in the first trimester is more accurate than any calculation.
Formula: conception ≈ last period + (cycle length − 14) days; due date = last period + 280 days
Worked examples
| Inputs | Likely conception date | Note |
|---|---|---|
| Last period 1 June 2026, 28-day cycle | Monday 15 June 2026 | conception about 15 June; due 8 March 2027 |
| Working back from a due date | Monday 15 June 2026 | the same dates in reverse |
| From a 12-week scan | Monday 15 June 2026 | scan dating overrides the remembered period |
FAQFrequently asked questions
Why is pregnancy dated from my last period?
Because that date is usually known, while the moment of conception is not. It means the first two weeks of a "pregnancy" happen before conception.
How accurate is a conception date?
It is an estimate. Ovulation shifts between cycles, and sperm can survive about five days, so conception may have happened anywhere in the fertile window shown.
What if my cycle is not 28 days?
Enter your own length. The second half of the cycle is fairly fixed at about 14 days, so a 35-day cycle ovulates around day 21, not day 17.
Which is more reliable, a scan or a calculation?
A first-trimester scan. If a scan date and a period date disagree by more than about a week, clinicians use the scan.
Can this tell me who the father is?
No. The window is wide and the estimate imprecise; only a DNA test can answer that.
Where these figures come from
- Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 — the BMR equation used here
- World Health Organization — Waist circumference and waist–hip ratio (2008 expert consultation) — waist-to-hip cut-offs
- Tanaka H, Monahan KD, Seals DR, "Age-predicted maximal heart rate revisited", J Am Coll Cardiol 2001 — 208 − 0.7 × age
- Nathan DM et al., "Translating the A1C assay into estimated average glucose values" (ADAG), Diabetes Care 2008 — eAG = 28.7 × A1c − 46.7
- Mosteller RD, "Simplified calculation of body-surface area", N Engl J Med 1987 — BSA = √(height × weight / 3600)
- Australian Department of Health and Aged Care — national health guidance
Last checked: September 2026. The equations are fixed by their published papers and do not change with tax years or regulations; the thresholds quoted in verdicts name the body that set them.