A “chemical pregnancy” refers to pregnancy which has only been detected by testing for the level of the pregnancy hormone hCG (a biological chemical). High-levels of hCG indicate a pregnancy. A chemical pregnancy may be detected two weeks after conception.
A “clinical pregnancy” means there are clinical signs of a foetus, which can be seen (as in an ultrasound test) or heard (for example, by listening to a foetal heartbeat). The earliest clinical signs of pregnancy are detectable at around five weeks pregnant, or one week after your period is late.