Yolk Sac Question (IVF related)

Baby_Bunny_Hopeful

My brother and SIL just had their first IVF and had three blood tests all with increasing betas!!!  Everyone was super excited... and then they had their first u/s yesterday.  They only saw a yolk sac, nothing inside.  According to the dr- she should be 6w2d, but sac was measuring 5+weeks (so a week behind).

They scheduled a repeat u/s for next week.... and the doctors said everything could still be fine??

Just wondering if anyone experienced this, what the outcome was and what exactly this all means for their pregnancy.

TIA!!  :-)