Conception Date Calculator
Estimate when conception likely happened — from your due date, last period, an ultrasound or a baby’s birth date — with the fertile-window range, the most likely days, implantation and an early-pregnancy timeline.
How it works
From a last period: conception ≈ LMP + (cycle length − luteal phase), i.e. around ovulation · from a due date: due date − 266 days · from an ultrasound: scan date − gestational age at the scan + 14 days · from a birth: birth date − (gestational age at birth − 14 days) · fertile window = conception − 5 to + 1 day
Conception happens at or just after ovulation, which is estimated as about 14 days (your luteal phase) before your next period was due — so roughly (cycle − 14) days after your last period began. A due date sits 266 days (38 weeks) after conception, an ultrasound gives the gestational age on the scan day (which counts from two weeks before conception), and a birth record gives the gestational age at birth, so each can be worked back to the same date. Because sperm can survive up to five days and the egg about a day, intercourse on any day of the fertile window could have led to the pregnancy, which is why this shows a range and the average chance for each day rather than a single certainty. The timeline then shows implantation, the missed period, the first heartbeat on ultrasound and the due date that follow from it.
Worked example
With a last period on June 1, 2026 and a 28-day cycle, ovulation — and most likely conception — falls around June 15, 2026, with the fertile window running June 10 to June 16 and a due date of March 8, 2027. Working back from that due date gives the same June 15. A baby born March 1, 2027 at 39 weeks 0 days also points to June 15, 2026 (259 days earlier).
Sources: ACOG Committee Opinion 700 — Methods for Estimating the Due Date · Wilcox, Weinberg & Baird (1995) — Timing of sexual intercourse in relation to ovulation, NEJM
Frequently asked questions
Can I know the exact day I conceived?
Rarely. Sperm can live up to five days and the egg about a day, so conception may happen days after intercourse. This gives the most likely date, not a certainty — a clinician can narrow it with an early scan.
Can I use this to prove timing for legal purposes?
No. It is an estimate from average cycle timing and should never be used to establish legal or medical facts such as paternity. Only a doctor with proper testing can determine those.
My cycles are irregular — is this still useful?
Less so. The estimate assumes ovulation about 14 days before your next period, which shifts with cycle length. If you had an early ultrasound, choose that method instead: it is usually more accurate than the last-period date.
How do I work out conception from a birthday?
Choose “Baby’s birth date” and enter the gestational age at birth if you know it. Conception is that many weeks, minus two, before the birth. Without it, 40 weeks is assumed, which can be off by two weeks or more.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.