I don't know whts it's short for tbh, but it's basically the growth hormone and when a lady is not pregnant they apparently lie between 0-5 when pregnant they rise daily, supposedyl doubling every 48 hrs, but like someone said all women ae different and doesn't necessarily do this. It can be a sign of twins if the levels are mega mega high, but not all the time.
The hormone stops doubling after the placenta has grown at abt 12 weeks, hence we enter the 'safer' period. Which is why some women who are 6months to full term often have neg pregnancy tests.