"yolk sac should be seen on transabdominal scanning when the mean sac diameter (MSD) is 20 mm or at a gestational age of 7 weeks and is usually seen endovaginally with an MSD of 8-10 mm or gestational age of 5.5 weeks."
"Yolk sac is the first anatomical structure identified within the gestational sac. It plays a critical role in embryonal development by providing nutrients, serving as the site of initial haematopoiesis, providing endocrine, metabolic and immunological functions and contributing to the development of fetal gastrointestinal and reproductive systems 2."