How the conception date is estimated
Conception happens at ovulation, within about a day of the egg being released, and ovulation in a textbook cycle falls 14 days after the period begins — 266 days before the due date. So from a due date the calculator subtracts 266 days; from a last period it adds 14 (adjusted for cycle length); from an ultrasound it back-calculates the LMP-equivalent start and adds 14. Because sperm survive up to five days in the reproductive tract, intercourse anywhere from five days before ovulation to the day after could have caused the pregnancy — a six-day window, not a single night.
The table walks through a due date of October 12, 2026.
| Step | Date | Note |
|---|---|---|
| Due date | October 12, 2026 | |
| LMP-equivalent start (due − 280) | January 5, 2026 | |
| Ovulation / conception (start + 14) | January 19, 2026 | Most likely day |
| Fertile window (conception − 5 to + 1) | January 14, 2026 – January 20, 2026 | Intercourse that could have led to it |
| Implantation (+ 6 to + 12 days) | January 25, 2026 – January 31, 2026 | hCG begins to rise |
| Earliest positive home test | January 29, 2026 | Sensitive tests; most by the missed period |
Cycle length moves the date
Ovulation is tied to the *next* period, about 14 days before it, so in a longer cycle it happens later. A 35-day cycle ovulates around day 21, a week later than a 28-day cycle. If you calculate from your last period, enter your usual cycle length; the table shows how much it matters.
| Cycle | Ovulation | Conception date | Due date |
|---|---|---|---|
| 24 days | Day 10 | March 11, 2026 | December 2, 2026 |
| 26 days | Day 12 | March 13, 2026 | December 4, 2026 |
| 28 days | Day 14 | March 15, 2026 | December 6, 2026 |
| 30 days | Day 16 | March 17, 2026 | December 8, 2026 |
| 32 days | Day 18 | March 19, 2026 | December 10, 2026 |
| 35 days | Day 21 | March 22, 2026 | December 13, 2026 |
From a birth date
For a baby already born, the birth date alone fixes conception only to within about two weeks, because babies arrive anywhere from 37 to 42 weeks. If you know the gestational age at birth (on the discharge summary or birth record), subtract it from the birth date to get the LMP-equivalent start and add 14 days. Without it, the calculator assumes 40 weeks and shows a ±2-week range.
| Born at | Conception | Note |
|---|---|---|
| 37 weeks | January 19, 2026 | Conception ~ 35 weeks before birth |
| 39 weeks | January 5, 2026 | |
| 40 weeks | December 29, 2025 | Assumed when the gestation is unknown |
| 41 weeks | December 22, 2025 | |
| 42 weeks | December 15, 2025 |
How precise each method is
The range shown with every result reflects the method. IVF is exact; tracked ovulation is almost as good; ultrasound and regular-cycle LMP dating are within a few days; irregular cycles and birth dates are much wider. If the question is which of two possible dates led to a pregnancy and they are less than about ten days apart, no calculation can answer it reliably — only DNA testing can.
| Known date | Typical accuracy | Why |
|---|---|---|
| IVF transfer date | Exact | Embryo age at transfer is known to the day |
| Tracked ovulation (LH test, temperature) | ± 1 day | Ovulation is observed, not inferred |
| First-trimester ultrasound | ± 3–5 days | Measures the embryo directly |
| Last period, regular cycle | ± 3–4 days | Assumes ovulation 14 days before the next period |
| Last period, irregular cycle | ± 1 week or more | Ovulation day varies |
| Birth date only | ± 2 weeks | Term spans 37–42 weeks |
Implantation and the first positive test
After conception the embryo travels to the uterus and implants six to twelve days later, most often at eight to ten. Only then does it begin producing hCG, the hormone pregnancy tests detect, so a test cannot turn positive before implantation. Sensitive tests can detect hCG two to three days after implantation — about ten days after conception at the earliest — and most pregnancies test positive by the day the period was due. Implantation bleeding, if it occurs, falls in the implantation window and is sometimes mistaken for a light period, which then throws off LMP-based dating.
Why the dates might not match your memory
Ovulation can shift by days from cycle to cycle with stress, illness, travel or breastfeeding, and a single cycle can be much longer than usual. Conception can also follow intercourse up to five days earlier, so the day you remember may be several days before the calculated conception date. If your ultrasound-based due date differs from the LMP one, the scan is more reliable and the conception estimate from it will be too.