No yolk sac strong heartbeat

Hi all,

We had our first ultrasound transabdominally at exactly nine weeks this past Thursday. The doctor was able to visualize a fetal pole with a strong heartbeat of 167 bpm however was not able to find the yolk sac. I have read on numerous blogs that by week 10 the placenta takes over and the yolk sac begins to disappear. Although I am only 9 weeks along. Just a bit worried that she was not able to visualize the yolk sac and was wondering if anyone else had a similar experience. I've found both good and bad posts regarding the yolk sac and am hoping that it was just not able to be seen through the abdominal us.