yoke sack vs placenta

I went for my first ultrasound yesterday and when the tech saw the baby she said I wasn't as far along as I thought I was because there is still a yoke sack.  She measured the baby and it was the right size for 9 weeks and 3 days. I thought I was 9 weeks and 2 days so it's right on!  She said it must be a big baby at only 7 and a half weeks.  I'm so confused.  The baby's heart beat was perfect and everything looks great.   I've read online that the placenta sometimes doesn't come until 10 weeks +.  Any of you moms/ expecting moms still have a yoke sack around 9 weeks?   Thanks so much!!!