When Can I Take a Pregnancy Test

Home pregnancy tests detect HCG (human chorionic gonadotropin), a hormone produced by the placenta starting roughly 6-10 days after conception. Sensitive urine tests can pick up HCG from about 11-14 days post-conception, but the most reliable result comes from testing on or after the first day of your missed period - by then HCG levels are usually high enough that even less sensitive tests will read positive. Blood tests at a clinic can detect HCG even earlier, around 6-8 days post-conception, but home urine tests are what most people use for the initial check.

The calculator works back from either your last menstrual period (LMP) or known ovulation date to give you the earliest reasonable urine test date, the earliest blood test date, and the most accurate urine test date (which is your missed period date or later). If you test too early and get a negative, that doesn't mean you're not pregnant - HCG may simply be below the detection threshold. The standard advice is to retest in 2-3 days; HCG roughly doubles every 48 hours in early pregnancy.

The calculation relies on the assumption that ovulation happens roughly 14 days before your next period would start. For a 28-day cycle, that's day 14 of the cycle (counted from day 1 of LMP). For a 30-day cycle, ovulation is around day 16. For a 26-day cycle, around day 12. This 'luteal phase' (the time from ovulation to next period) is more consistent than the 'follicular phase' (LMP to ovulation), which is why the calculation works backwards from the expected next period rather than forwards from LMP.

If your cycles are very irregular, the LMP-based calculation is less reliable. People with PCOS or other conditions causing variable cycle length might find the ovulation-date input more useful if they've been tracking with ovulation predictor kits, basal body temperature, or fertility awareness. The [ovulation calculator](/ovulation-calculator) helps establish ovulation timing if you haven't yet.

Tests are rated by their HCG sensitivity, measured in mIU/mL (milli-international units per millilitre). The most sensitive home tests detect from 10 mIU/mL; mid-range tests detect from 25 mIU/mL; basic supermarket tests typically need 50 mIU/mL or higher. Earlier testing requires a more sensitive test. By the day of your missed period, HCG is typically 25-100 mIU/mL, so any standard test should pick it up; testing 3-4 days before your missed period might require a 'first response' or similar early-detection test.

False negatives are common when testing early and become rare after the missed period date. False positives are very rare in modern tests but can occur with certain medications (HCG-containing fertility treatments, some cancer treatments) or in the days following a miscarriage when HCG is still clearing. A faint line is still a positive line; HCG concentration affects line darkness but presence is binary. A positive result should be followed up with a GP or midwife appointment within a few weeks to confirm and start antenatal care.