1. Home pregnancy tests use antibodies that bind to hormones in a pregnant woman’s urine. The antibodies have dye molecules attached to them, which create a visible line on the test.
2. The hormone being detected is called human chorionic gonadotropin, or HCG, which the placenta secretes once a fertilised egg has implanted. It is detectable from the day the woman’s period is due.
3. Although HCG is specific to humans, other vertebrates have similar gonadotropin hormones that regulate their reproductive cycle. HCG collected from humans is used by vets to induce ovulation.
4. But it isn’t only horses that HCG can affect. In the 1960s, clinical pregnancy tests involved injecting a woman’s urine into the leg of a live African clawed frog. If the frog laid eggs the next day, the woman was pregnant.