Pregnancy test timing.

Pregnancy Test Timing Calculator: how it works

hCG first appears in urine about 6 to 12 days after ovulation, but the NHS says most home tests are reliable from the first day of a missed period. Enter your ovulation date and this calculator shows the implantation window in days past ovulation (DPO), your expected period date, and the date from which testing is most reliable. If you ovulated on 1 October 2026, implantation is expected 7 to 13 October and a test is most reliable from 15 October.

In a study of 221 women collecting daily urine samples, Wilcox and colleagues (1999) found that among pregnancies lasting at least six weeks, hCG first appeared 6 to 12 days after ovulation, and 84% of implantations happened on day 8, 9 or 10. The calculator shows 6 to 12 DPO as the implantation window and 8 to 10 DPO as the most common days, counting the ovulation day as DPO 0. That research used a sensitive laboratory measure of urinary hCG; a home test needs more hCG than that to show a positive.

The expected period is the ovulation date plus the number of days you enter (14 by default; the NHS says ovulation is about 12 to 16 days before the next period). The NHS also says most tests work from the first day of a missed period, that very sensitive tests may work earlier, and that if you do not know when your period is due, you should test at least 21 days after you last had unprotected sex. Ovulation dates from apps and temperature charts are estimates, so a day or two of error is normal.

Worked example. Ovulation on 1 October 2026 (DPO 0). The implantation window is 6 to 12 DPO, so 7 to 13 October; the most common days are 8 to 10 DPO, 9 to 11 October. With 14 days from ovulation to period, the expected period is 15 October, which is when testing is most reliable. If unprotected sex was on 29 September, 21 days later is 20 October. On 7 October the calculator would show 6 DPO, inside the implantation window, and 8 days to go.