I think it's easier to tell when your beta and progesterone are taken earlier on. They are supposed to take it and then take it about 48 hours later? Did they not check yours twice? Anyway, I already had had 3 kids before having the twins and I knew the numbers were off.
4w 2d = 883 progesterone = 22
4w 4d = 1912
5w 3d = 16,500
Beta is supposed to double every 48 hours, sometimes a little less sometimes a little more than double. It does depend on the person. I found out at 7w6d that we were having twins. Based on my beta, it would have been about 34,000 at 5w5d and 67,000 at 6w0d.
It was the blood draw from my first prenatal visit so I don't have anything to compare it to. I myself am a fraternal twin and have uncles who are also twins so it runs in my family as well as DH who has 3 sets of twins on his side. Thank you Stef110 for commenting .