Quoting Red Bottom:" A typical yolk sac at 5 weeks would be around 11mm...."
Tell that to my doctor and the placenta the examined..
By week 5 the placenta starts working
"In the meantime, the primitive placenta and umbilical cord, which deliver nourishment and oxygen to your baby, are already on the job."
"Now comes an exciting time of rapid growth! The amniotic sac, amniotic cavity and yolk sac are developing. The placenta is beginning to form now too."
according to Mayo clinic the placenta starts to form in week 4
So please don't tell me what you think you know. I have experienced it in December.