I cant say that I'm suprised at how baffling early she is announcing this pregnancy. For those who dont know, the very earliest that one can have a visible ultrasound (for something literally the size of a kidney bean) is 8 weeks. Robyn would NOT waste the opportunity to post that so judging by the August statement, and the fact that we only have a pregnancy test, im going to assume shes only 4 weeks pregnant. (and again, for those who dont know, "4 weeks" is sort of a misnomer. its technically just 2 weeks out from conception and when the hcg levels are high enough to give a positive result on a urin based pregnancy test)
Tl:dr At best Robyn has a 2 week old collection of cells that might become a viable embryo. at worst this is a chemical pregnancy and she is only a few days late from her last period.