How conception date is estimated
Conception – the moment sperm fertilises an egg – happens within about 12–24 hours of ovulation. So estimating conception is really estimating ovulation. In a textbook 28-day cycle that is day 14, which is why conception is placed two weeks after your LMP, or 266 days before your due date.
The fertile window explained
Sperm can survive in the female reproductive tract for up to five days, while an egg survives for about a day. That creates a six-day fertile window: the five days before ovulation plus ovulation day itself. A landmark study of couples trying to conceive found that nearly all pregnancies came from intercourse within this window, with the highest chances in the two days before ovulation.
| Intercourse timing | Chance it leads to pregnancy |
|---|---|
| 5 days before ovulation | Low |
| 3–4 days before | Moderate |
| 1–2 days before | Highest |
| Ovulation day | High |
| Day after ovulation | Very low |
From conception to implantation
After fertilisation the embryo travels down the fallopian tube, dividing as it goes. It reaches the uterus as a blastocyst and implants into the lining 6–12 days after ovulation (most often days 8–10). Only after implantation does the body start producing hCG, the hormone detected by pregnancy tests. That is why a test taken a few days after intercourse is always negative.
Can a calculator pinpoint the exact day?
No – and it is worth knowing why before drawing conclusions about when or with whom a pregnancy began. Ovulation can shift by several days in any given cycle, sperm can live for days, and even first-trimester scans carry a margin of error of about ±5 days. A calculator gives a likely window, not proof of a specific date. If this matters for personal or legal reasons, speak to your doctor; only a DNA test can establish paternity.
Frequently asked questions
How do I know when I conceived?
The most likely day is your ovulation day – about 14 days before your next expected period, or 266 days before your due date. Conception could have followed intercourse on any of the five days before that.
Can I get pregnant right after my period?
It is possible if you have a short cycle. With a 21–24-day cycle you may ovulate around day 7–10, and sperm from intercourse near the end of your period could still be alive.
How many days after intercourse does conception happen?
Anywhere from a few hours up to about five days, depending on how long before ovulation intercourse took place.
Is conception date the same as the first day of pregnancy?
Medically, pregnancy is dated from the LMP, about two weeks before conception. Conception usually falls at 2 weeks 0 days of gestational age.
Can an ultrasound tell the exact conception date?
An early scan can estimate it to within about 5 days. It cannot identify the exact day or which act of intercourse led to the pregnancy.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.
Sources & further reading
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21.
- Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999;340:1796–9.
- American College of Obstetricians and Gynecologists (ACOG). Committee Opinion No. 700: Methods for Estimating the Due Date. acog.org