A lot of drs rx progesterone to prevent early loss. I am sorry, I don't know anything about a 7day embryo. I have never done IVF and only heard of 3 & 5 day embryos.
Normally, when you O, the follicle that releases the egg produces progesterone to sustain the pregnancy through the first trimester. If you went through ivf, you actually never O'd since the eggs were retrieved. Thus, you need to inject progesterone or else your body will miscarry. I only took estrogen through my 2ww after ivf.
I don't know much about 7 day embryo.