16 gestational sac can my pregnancy progress normally?

Hi everyone, so I'm 6 weeks and 6 days pregnant I went in today for my first ultrasound this is my first pregnancy and my doctor told me I had a big gestational sac 16 cm as opposed to the 16 mm it should be...Does anyone know anything about this or experienced anything similar? What could this mean. I'm waiting for an appt in another clinic but until then I wanted to hear from possibly experienced people. Also she told me it was too early for a heartbeat when I asked.