A brand-new placenta starts pumping out hCG, or human chorionic gonadotropin, a few days after the budding embryo implants in the uterine wall. Although the level of hCG will initially be quite low (it's just starting to show up in your system, after all) — it'll soon begin to soar, doubling every 30 hours (give or take). The rapid increase peaks somewhere between seven and 12 weeks after the last menstrual period (LMP), and then starts to decline.
While it might sound like a predictable increase, when it comes to producing predictable levels, it's not even close. Experts have found a huge variation in hCG levels on even the first missed day of a woman's period. Some women have no measurable hCG (in other words, a level of 0 mIU/ml), while others have readings over 400. Either way, this is perfectly normal and has to do, in part, with exactly how long it took your egg to get fertilized, make the trip down to the uterus, and get snuggled in; some embryos are simply zippier than others. It's also perfectly normal for that wide range in levels to persist throughout pregnancy.