When can a test detect pregnancy?
Pregnancy tests detect hCG, a hormone made by the placenta once the embryo has implanted in the uterus. Implantation happens 6 to 12 days after ovulation, most often around days 8 to 10, and hCG then roughly doubles every two to three days. A test turns positive only when the level reaches its detection threshold.
The dates
- Ovulation ≈ last period + cycle length − 14 days
- Implantation window = ovulation + 6 to 12 days
- Earliest sensitive test ≈ ovulation + 10 days
- Reliable home test = the day the next period is due
Worked example
Your last period started on October 14, 2026 and your cycle is 28 days.
- Ovulation: around October 28.
- Implantation: between November 3 and November 9.
- A sensitive early test could turn positive from about November 7, though many pregnancies are not yet detectable then.
- A home test is reliable from November 11, the day your period is due, 8 days from November 3.
Home tests and blood tests
| Test | Typical sensitivity | When |
|---|---|---|
| Standard urine test | 20 to 25 mIU/mL | from the missed period |
| Early-result urine test | about 10 mIU/mL | a few days before the missed period |
| Quantitative blood test | about 1 to 5 mIU/mL | about 10 days after ovulation |
The packaging of early tests often claims accuracy of 99% “from the day of the expected period”; several days earlier, the chance of a false negative is much higher.
Tips for an accurate result
- Use first-morning urine, which has the highest concentration of hCG.
- Follow the reading time on the instructions; a faint line that appears much later can be misleading.
- A faint positive line within the reading time usually means pregnancy, since tests react only to hCG.
- If the result is negative and your period does not come, test again in two or three days.
False positives and negatives
False negatives are common when testing too early or with diluted urine. False positives are rare, but can follow a very early pregnancy loss, recent hCG injections for fertility treatment, or certain medical conditions. A blood test and an ultrasound confirm the result.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.