Pregnancy: are test accurate?

How accurate are home pregnancy tests? Is it possible to be pregnant even though the test is negative on the day my period is due? And is it possible that a positive pregnancy test could be wrong?

Dr Fred Kavalier answers your health question:

Home pregnancy tests are designed to detect tiny amounts of a hormone called human chorionic gonadotrophin (hCG) in the urine. The hormone is produced by the placenta, and it begins to appear in the mother's urine as soon as the developing embryo is implanted in the wall of the uterus - hCG cannot appear in the mother's urine until the embryo is attached to the uterus. The embryo attaches to the wall of the uterus between 6 and 12 days after ovulation. So if a woman has a 28 day menstrual cycle, she usually will ovulate on day 14 of her cycle.

If she falls pregnant, the earliest time that hCG is present is about a week before her period is due. A small number of pregnancies will not have implanted by the first day of the missed period, and in these cases a urine pregnancy test will not yet be positive. A positive pregnancy test is extremely unlikely to be incorrect if you follow the instructions correctly. If you have had an injection of hCG (for fertility treatment) this could cause a test to be positive in a woman who is not pregnant.

Please mail your questions for Dr Fred to health@independent.co.uk. He regrets that he is unable to respond personally to questions.