I need some answers

I call my ob cause when I went in for my 1st check up she told me it was two sac and one was empty. Which I get . But as I was leave said well you are at high risk . Then I call back today to ask her nurse what she mean . She said she really didn't know and they was send me for a level 2 us when I'm close to 20 or 27 weeks . Which this sac is still grown since I 1st went to the er back in dec . I when for my 1st ob check up this month . So I ask if the sac don't leave and keep grow then what going happen . She told me she didn't know . But it could put my other baby at risk . So I might have to get rid of the baby at 20 some thing week if it continued to grow. Why is and do I need to see a other doc . But she told me I won't get no more us at all. In til it time to check the sex of the baby