Conception Date Calculator
Work out when you conceived. Enter your last period, your due date, or your positive test date — we'll estimate the conception and implantation windows to the day.
Calculate your conception date
Work backward from the date you know.
Cycle length adjustment (for irregular or long/short cycles)
Your fertile window
Conception most likely happened somewhere in this seven-day span — peaking on the highlighted day.
Implantation & early pregnancy timeline
Implantation happens 6–12 days after conception. Home tests usually detect pregnancy around 14 days after conception.
Confirm your pregnancy with an early scan
An early pregnancy scan from 6 weeks confirms location, heartbeat and viability — often before most clinics will see you.
How conception date is estimated
Conception happens when a sperm fertilises an egg — and an egg is only viable for about 24 hours after ovulation. In a textbook 28-day cycle, ovulation happens on day 14, so conception most likely happened on or very close to that day. Add in the fact that sperm can survive inside the body for up to five days, and the fertile window stretches to roughly six days — the five days before ovulation plus ovulation day itself.
The calculator works backward from whichever date you know. If you enter your last period, it adds 14 days to estimate ovulation, adjusting for cycle length. If you enter your due date, it works backward 266 days (38 weeks from conception). If you enter your positive pregnancy test date, it subtracts roughly 14 days — the typical gap between conception and a test turning positive.
All three methods produce a single "most likely" conception date, but it's more accurate to think of it as a window. You'll see the central date plus a ±3 day band showing the full fertile window around it.
Conception vs. implantation
These aren't the same event. Conception is when sperm meets egg — in the fallopian tube, usually within 24 hours of ovulation.
Implantation happens 6–12 days later, when the embryo attaches to the uterus lining. Some women experience light bleeding ("implantation spotting") around this time, though most don't notice anything.
Home pregnancy tests only become reliable after implantation, when hCG reaches detectable levels.
How accurate is a conception date?
The short answer: accurate to within a few days at best. Here's what affects it.
Frequently asked questions
Can I know exactly what day I conceived?
Not usually. Sperm can survive inside the body for up to five days, and the egg is viable for about 24 hours after ovulation. That gives a six-day fertile window during which conception could have happened. The calculator gives you the most likely single day, but the true answer is always a range. IVF is the only scenario where conception is known to the day.
My cycle is irregular. Is the result still useful?
Use the cycle-length adjustment in advanced options to improve the estimate, but recognise that irregular cycles make ovulation timing genuinely unpredictable. The calculated date should be treated as a rough guide rather than a precise answer. An early pregnancy scan will give you a more reliable answer than any date-based method.
Why does the calculator sometimes give a date I know couldn't be right?
Almost always because the underlying cycle assumption doesn't match reality — you ovulated earlier or later than day 14, your last period date was different from what you remembered, or implantation spotting was mistaken for a period. If the calculator's answer disagrees with what you know, your knowledge wins. The calculator applies a standard formula that doesn't know anything about your individual cycle.
How soon after conception can I take a pregnancy test?
Home pregnancy tests typically become reliable about 14 days after conception — around the time your next period would have been due. Testing earlier risks a false negative because hCG levels haven't yet reached detectable thresholds. More sensitive early-detection tests can work from about 10 days after conception, but accuracy improves significantly with waiting.
What about IVF — how do I find my conception date?
You already know it. For IVF, fertilisation happens in the laboratory on a known day — typically the same day as egg retrieval. Conception is that date. If you had a day-3 transfer, the embryo was conceived three days before transfer; for a day-5 blastocyst, five days before. Calculators aren't needed for IVF pregnancies.
Can an early pregnancy scan confirm conception date?
It can confirm gestational age very precisely — which is closely related. A scan between 7 and 13 weeks measures your baby's crown-rump length and dates the pregnancy to within three to five days. From that, working back 14 days gives you a conception date accurate to roughly a week — typically more reliable than LMP-based calculators.
Book an early pregnancy scan
From 6 weeks, an early scan confirms location, heartbeat and viability — reassurance when it matters most. Same-week appointments at our Kensington clinic.