Find your estimated conception date by working backward from your due date (EDD) or forward from your last menstrual period.
Your conception date is estimated by working backwards from a known date. From a due date, subtract 266 days (38 weeks). From your last period, add about 14 days. Because ovulation timing varies between cycles, the result is an estimated window of a few days rather than one exact date. Ultrasound-based dating gives the most reliable estimate.
| Event | Date |
|---|
Because conception cannot be observed directly, it is estimated backwards from a date you know — usually the due date, last period, or an ultrasound.
| Starting Point | Method | Reliability |
|---|---|---|
| Due date (EDD) | Subtract 266 days | Good — reflects your dating |
| Last period (LMP) | Add ~14 days | Good if cycles are regular |
| Ultrasound | Back-calculate from gestational age | Most reliable |
| IVF / IUI | Transfer or insemination date | Very precise (known timing) |
A conception "date" is really a short range for two biological reasons: sperm can survive in the body for up to about five days, and the egg lives for roughly 12–24 hours after ovulation. So conception could occur any time within that fertile span. This is why calculators estimate a likely date with a margin of a few days on either side.
Once conception is estimated, the pregnancy timeline follows a predictable pattern (measured as weeks since your last period, which is about two weeks before conception).
| Time After Conception | Approximate Stage |
|---|---|
| ~1 week | Fertilised egg implants in the uterus |
| ~2 weeks | Missed period; pregnancy tests become positive |
| ~4 weeks | Around 6 weeks "pregnant" by LMP dating |
| ~8 weeks | Major organs forming; often the first ultrasound |
| ~38 weeks | Estimated due date |
Conception typically occurs around ovulation — about 14 days before the next expected period. Working backward from an EDD: EDD − 266 days = approximate conception date (or EDD − 280 + 14 = ovulation day).
Conception is the moment of fertilisation — when a sperm cell successfully penetrates and fuses with the egg. This occurs in the fallopian tube, typically within 12-24 hours of ovulation. Because sperm can survive in the female reproductive tract for up to 5 days, conception may occur days after intercourse. An embryo typically implants in the uterine wall 6-10 days after fertilisation.
| Known Information | How to Calculate Conception Date |
|---|---|
| Due date (EDD) | Subtract 266 days (38 weeks) — conception to birth is approximately 266 days |
| Last menstrual period (LMP) | Add 14 days for 28-day cycle (adjust for longer/shorter cycles) |
| Ultrasound gestational age | Subtract gestational age in days from scan date, then add 14 days |
| Days After Conception | Development Stage |
|---|---|
| Day 0 | Fertilisation in fallopian tube |
| Days 3-4 | Blastocyst travels to uterus |
| Days 6-10 | Implantation in uterine wall (pregnancy officially begins) |
| Day 10-14 | hCG rises — pregnancy test becomes positive |
| Week 4-5 (from LMP) | Missed period; positive home pregnancy test |
| Week 6 (from LMP) | Heartbeat detectable on transvaginal ultrasound |
When exactly does conception occur?
Conception (fertilisation) occurs when a sperm penetrates the egg, typically in the fallopian tube within 12-24 hours of ovulation. Because sperm survive for up to 5 days, conception may happen several days after intercourse. The resulting fertilised egg (zygote) travels to the uterus over 3-5 days, then implants 6-10 days after fertilisation — when pregnancy is officially established.
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from your estimated due date. Example: due date 1 March 2027 → conception approximately 8 June 2026. This gives the approximate fertilisation date. Conception could have occurred 1-5 days before this if the date is derived from ovulation timing, due to sperm survival.
When does a home pregnancy test turn positive?
Home pregnancy tests detect hCG (human chorionic gonadotropin), which rises from implantation (approximately 6-10 days after fertilisation). Most sensitive tests can detect pregnancy approximately 10-12 days after fertilisation — around the time of the missed period. Testing too early (before 10 days post-ovulation) often gives false negatives even in true pregnancies.
How do you calculate conception date?
Work backwards from a date you know. From your due date, subtract 266 days (38 weeks). From your last period, add about 14 days to reach the likely ovulation and conception time. Ultrasound dating is the most accurate. Enter your date in the calculator and it estimates your conception window.
How do I calculate conception from my due date?
Subtract 266 days (about 38 weeks) from your estimated due date. For example, a due date of 1 December gives an estimated conception around 10 March. Because ovulation timing varies, treat this as a window of a few days rather than one exact date.
Can a conception calculator tell me exactly when I conceived?
No. It estimates a likely window of a few days, not a precise moment. Sperm can survive up to five days and the egg for about a day, so conception could occur anywhere in that span. Ultrasound dating narrows the estimate but still gives a range.
Can a conception calculator determine paternity?
No. Because the fertile window spans several days, a calculator cannot identify which of two partners is the biological father. Only a DNA paternity test, carried out by a qualified provider, can determine this reliably. A conception estimate should never be used to draw conclusions about paternity.
How accurate is a conception date from ultrasound?
Ultrasound dating, especially in the first trimester, is the most accurate way to estimate conception, usually within a few days. Early scans measure the baby directly rather than relying on cycle dates, which is helpful if your periods are irregular or you are unsure of your last period.
What is the difference between conception and implantation?
Conception (fertilisation) is when sperm meets egg, usually around ovulation. Implantation is when the fertilised egg attaches to the uterus lining, about 6–10 days later. Pregnancy tests detect hormones produced after implantation, which is why they turn positive around the time of a missed period.
Why is pregnancy dated from the last period, not conception?
By convention, pregnancy is counted from the first day of your last period because that date is easier to know than the exact moment of conception. This means you are considered about two weeks "pregnant" at conception, and 40 weeks from the last period equals about 38 weeks from conception.