How the Pregnancy Test Calculator works
Home pregnancy tests detect the hormone hCG, which the body starts producing after a fertilised egg implants in the uterus. Implantation usually happens 6 to 12 days after ovulation, and hCG then needs a few more days to reach a level a urine test can pick up. That is why testing too early can give a negative result even if you are pregnant.
Using your last period and cycle length, the calculator estimates when you ovulated and when your next period is due, then shows three dates: the earliest day a sensitive test might work, the day your period is due (when most tests become reliable) and a week later (when the result is most accurate). If you only know when you had unprotected sex, it uses the common guidance of testing about three weeks afterwards.
When to test: a quick guide
| Timing | How reliable? |
|---|---|
| Before ~10 days after ovulation | Too early for most pregnancies to show |
| A few days before the period is due | Possible with sensitive “early result” tests, but negatives are common |
| The day your period is due | Most tests are reliable from here |
| A week after the missed period | Very reliable — repeat a negative test now |
| 21 days after unprotected sex | Reliable if you don’t know when your period is due |
Tips for an accurate result
- Read the instructions — tests differ in how long to wait and how to read the result.
- For early testing, use first-morning urine and avoid drinking a lot of fluid beforehand.
- Read the result within the time on the pack; lines that appear later may not be meaningful.
- A faint positive line usually still means hCG was detected. Testing again in 48 hours should show a darker line.
- Check the expiry date — expired tests can be unreliable.
After a positive test
Congratulations — use the Pregnancy Calculator to estimate your due date and how many weeks pregnant you are, and contact a doctor or midwife to arrange your first appointment. If you have pain on one side, bleeding, shoulder-tip pain or feel faint, seek medical advice promptly.
If you are trying to conceive and want to time things for next cycle, the Ovulation Calculator shows your fertile window, and the Implantation Calculator explains the days in between.