Confused

My last period was Jan 17 and as per LMP, i am now 7weeks 6days preggy.. i experienced super little brown discharge nung feb 21 (5th week) once lang but naforce kme ni OB na magpa-TVS.. sa result nun nung feb 25, malabong picture ng gestational sac pa lang with 5weeks MSD so advised ni OB, TVS again after 2weeks. I had the 2nd one yesterday and it appeared na clear na ung picture ng gestational sac ko with 6weeks 0day pero ala pang yolk sac.. bakit ganun, 5weeks per 1st TVS then 2weeks after, 6weeks plang naman lumabas sa 2nd TVS?