How pregnancy tests work
Home pregnancy tests (often sold in India as pregnancy kits or cards) detect human chorionic gonadotropin (hCG), a hormone made by the developing placenta. hCG only appears after the embryo implants in the uterus, 6–12 days after ovulation. Levels then roughly double every two to three days in early pregnancy.
Most home tests detect hCG at 20–25 mIU/mL. On the day your period is due, most – but not all – pregnant women have reached that level. A blood test (serum beta hCG) can pick up lower levels a few days earlier.
The best time to test
| Timing | Reliability |
|---|---|
| Less than 10 days after ovulation | Very likely to be negative even if pregnant |
| 3–4 days before missed period (sensitive test) | Detects some pregnancies; negative doesn’t rule it out |
| Day of missed period | Good – the day most kits are designed for |
| One week after missed period | Very reliable |
| 21 days after unprotected sex (period date unknown) | Very reliable |
Use first-morning urine when testing early, as it is the most concentrated. Don’t drink lots of water just before testing.
Negative but no period?
You may have ovulated later than usual, meaning the test was simply too early. Retest in 3–7 days. Stress, illness, travel, sudden weight change, thyroid problems, PCOS and stopping hormonal contraception can all delay a period. If your period is more than a week late and tests remain negative, see a doctor.
Faint lines and evaporation lines
A faint line that appears within the time window stated on the instructions usually means hCG was detected – it is often just early. A colourless, grey line that appears after the reading window may be an evaporation line. Read the result at the stated time and retest in two days if unsure; a true positive gets darker as hCG rises.
See a doctor urgently if you have a positive test with one-sided abdominal pain, shoulder-tip pain, dizziness or heavy bleeding. These can be signs of an ectopic pregnancy, which is a medical emergency.
What to do after a positive test
- Work out your dates with the due date calculator.
- Start (or continue) folic acid and book a visit with a gynaecologist or your nearest government health centre.
- In India, register your pregnancy at an Anganwadi centre or PHC to access free check-ups under PMSMA (9th of every month) and benefits such as JSY and PMMVY.
- Avoid alcohol, smoking and any medicines not approved by a doctor.
Frequently asked questions
How soon can a pregnancy test detect pregnancy?
The earliest is around 10 days after ovulation with a sensitive test, but many pregnancies won’t show until the day of the missed period or later.
How many days after intercourse should I take a pregnancy test?
For a reliable result, wait until your period is due. If you don’t know when that is, test 21 days after unprotected sex.
Can a pregnancy test be wrong?
False negatives are common when testing early. False positives are rare but can occur after a recent miscarriage or abortion, with fertility injections containing hCG, or with certain medical conditions.
Is a blood test better than a urine test?
A blood beta hCG test is more sensitive and gives a number, which helps doctors track early pregnancy. It can detect pregnancy a few days earlier than a urine test.
Can I take a pregnancy test at night?
Yes, especially after your period is late. Early on, first-morning urine is better because it is more concentrated.
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, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999;340:1796–9.
- NHS (UK). Pregnancy tests; Ovulation; Your pregnancy and baby guide. nhs.uk
- Ministry of Health & Family Welfare, Government of India. Maternal health guidelines, PMSMA and Anemia Mukt Bharat. nhm.gov.in