There are two types of test:       

Blood Tests
Only blood tests can measure the pregnancy hormone (HCG) in the blood and date your pregnancy.  They have to be done at the doctor’s, but if your surgery has a long waiting list for an appointment, take a home test first to void time-wasting.

Urine Tests
These can be bought in chemists without a prescription, and measure the levels of HCG in your urine, which is not present in women who aren’t pregnant.  This hormone appears as soon as the embryo is embedded in the lining of the uterus, around 10 days after fertilisation, so you will need to wait at least 10 days after having sex.  Pregnancy tests bought over the counter are usually around 99% reliable and give you an easy-to-interpret result in minutes – some brands even show ‘pregnant’ or ‘not pregnant’, so there’s no room for confusion!  It’s best to test in the morning, when your urine is more concentrated.