They check for the rate of increase in human chorionic gonadotropin (HCG), a hormone produced by the placenta after implantation. A number above 50 shows implantation has happened, but the rate of increase is even more important (should be doubling approximately every 48-72 hrs) as an indicator of an embryo that “sticks”.
Best of luck, @ginny_e!
Thanks all! I’ve been having some weird (but not unexpected) diabetes effects, I’m sure you all have been through them! My basal rate as of Monday through yesterday has progressively increased, it’s the highest I have ever had it. The other thing I thought was weird, although not bad, is how a blood sugar over 130 now makes me feel like I have a crazy high blood sugar. Can’t say I’m upset about it, it is actually a bit of a relief, I was worried that keeping my blood sugars lower would make me feel low all the time. I am getting much, much better at pre-bolusing (15-20 minutes before I eat), and I must be getting better at carb counting, I’ve also been very careful about when and how many carbs I eat. Hope this continues! Fingers crossed, it is still very very early!
I’ve also discovered how much I love cut strawberries soaked in coconut milk 
I found that 130s were feeling like 200s when I started low carb and started consistently running near normal bgs. It’s kind of nice because it helps me remember to check my sensor or meter and treat more quickly so I can keep my average nice and normal.
Hang in there. It’s an adventure.
So about a week ago I had my first ultrasound and it did not go well. The embryo was only showing about 6 weeks of growth (I estimated I should have been 8ish), and my doctor told me I would most likely miscarry (I haven’t yet). I’m due to go for another utlrasound in a couple of days to see if there has been any growth on the chance that my dates are off, but ugh I’m worried. My hormone levels keep increasing like the pregnancy is progressing, and my blood sugars have been great. I’m really hoping that I miscalculated but just don’t think that is so. The ultrasound also showed that I have ovulated 2x, I’m not sure if that would throw off dates, or the fact I got 2 colds. Sad.