Estimating the Date of Conception
Conception happens when a sperm fertilises an egg, which can only occur in the roughly 12–24 hours after ovulation. So estimating conception means estimating ovulation.
In a textbook 28-day cycle, ovulation falls on day 14 — fourteen days after the first day of the last period. Working from a due date instead, conception is 266 days before it, since the due date is 280 days from the LMP and conception is 14 days after that.
Conception ≈ LMP + 14 days or Conception ≈ Due date − 266 days
Why Cycle Length Matters
The second half of the cycle — the luteal phase, from ovulation to the next period — is fairly constant at about 14 days. The first half varies. So in a 35-day cycle, ovulation happens around day 21, not day 14, and conception is a week later than the standard formula suggests.
| Cycle length | Approximate ovulation day | Shift vs. a 28-day cycle |
|---|---|---|
| 21 days | Day 7 | 7 days earlier |
| 24 days | Day 10 | 4 days earlier |
| 28 days | Day 14 | — |
| 32 days | Day 18 | 4 days later |
| 35 days | Day 21 | 7 days later |
The Fertile Window
Conception is possible from intercourse over a span of about six days, not one. Sperm survive up to five days in fertile cervical mucus; the egg survives about 12–24 hours after release. The window therefore opens roughly five days before ovulation and closes about a day after it.
| Day relative to ovulation | Approximate chance of conception |
|---|---|
| 5 days before | ~8% |
| 4 days before | ~15% |
| 3 days before | ~23% |
| 2 days before | ~27% |
| 1 day before | ~33% |
| Ovulation day | ~25% |
| 1 day after | ~8% |
| 2+ days after | Close to zero |
Note the peak is the day before ovulation rather than the day itself — sperm need time to reach the fallopian tube and undergo capacitation, so being present in advance beats arriving on the day.
What Happens After Conception
| Days after conception | Event |
|---|---|
| 0 | Fertilisation in the fallopian tube |
| 1–4 | The zygote divides while travelling toward the uterus |
| 5–6 | Blastocyst stage; arrival in the uterus |
| 6–12 | Implantation in the uterine lining; hCG production begins |
| 10–14 | A blood test can detect pregnancy |
| 14–18 | A home urine test becomes reliable |
Around a third of women experience light implantation spotting, which is sometimes mistaken for a light period and can throw off LMP-based dating.
How Accurate Is This?
Less accurate than most people expect. Ovulation varies between cycles even in regular ones, and studies using hormonal monitoring find that only about 30% of women have their fertile window entirely within the days predicted by the standard formula.
| Method | Accuracy of the conception date |
|---|---|
| IVF procedure | Exact — the date is known |
| LH surge test | ±1–2 days |
| Basal body temperature tracking | ±2–3 days, and only confirms after the fact |
| First-trimester ultrasound | ±5–7 days |
| LMP calculation | ±1–2 weeks |
An Important Caution
This calculator gives a probabilistic estimate over a range of days. It cannot establish the date of conception precisely, and it should not be used to determine paternity, to establish a legal timeline, or to draw conclusions about who was present on a particular date. The fertile window spans about six days, and ultrasound dating carries a margin of roughly a week either side. Only DNA testing can establish paternity.
Frequently Asked Questions
Can I conceive outside the fertile window?
Effectively no — the egg is only viable for about a day. But the window's timing can shift unexpectedly if ovulation is earlier or later than usual, which is why cycle tracking is unreliable as contraception.
Why does my conception date differ from what my doctor said?
Clinicians usually date from an ultrasound rather than the LMP, and they normally report gestational age rather than a conception date. Converting between them shifts the date by two weeks.
Does implantation bleeding change the calculation?
It can, indirectly. If spotting was mistaken for a period, the LMP used will be wrong by about a month. An early scan resolves this.
Is the conception date the same as the ovulation date?
Practically yes. Fertilisation occurs within about 24 hours of ovulation, so the two are treated as the same day for dating purposes even when intercourse occurred days earlier.