Chemical pregnancy is a form of very early miscarriage.
It is where a pregnancy is detected by HPT before your period is due, yet your period arrives later on, your baby is therefore lost before the heartbeat is detected.
We all know the baby's heartbeat can only be detected by ultrasound at around 6 gestational weeks.
The gestational age method is to start counting your pregnancy from LMP - last menstrual period, so by the time you actually conceive the baby, you are already 2-weeks' pregnant, and by the time your next period should be due, you are 4-weeks' pregnant.
So 6 gestational weeks = 2 weeks past your period due date. In plain language, if you test +HPT before your period is due, then your period arrives on time or within 2 weeks of its due date, then you have a chemical pregnancy.
This type of miscarriage is quite common. I suppose in old days, women didn't hang in there dying to test like we do now, haha, so most of the chemical pregnancy wouldn't be detected. It was just a late period for them. These days, the tests get more and more sensitive, and we do heck a lot more HPTs before the period is even due. This causes more chemical miscarriages being picked up.
You can say this is the down side of using highly sensitive HPTs. For some, they just NEED to know. For others each loss is a loss. It takes its toll. Ignorance could be a bliss for some.
Something to think about when you choose to buy super sensitive HPTs. :)