Twins?

Samantha

I went for an ultrasound 7 weeks. One baby. I went for my blood work for the genetics test at 10 weeks 2 days. I just got my results back at 12 weeks and it says both twins are girls….little confused as I only seen one baby at 7 weeks and again at 11 weeks. Is it possible there’s two and we’re just missing it? Why would the genetics test come back that there’s two if there wasn’t two on the ultrasound?