First ultrasound with 2nd bub

Because I have PCOS my periods were all over the place to begin with so my doctor was unable to calculate an EDD so I needed a dating scan. It revealed only a sac and nothing else and calculated to be about 5 weeks 2 days. I'm currently 9 weeks along. I've had mixed reviews of my ultrasound from friends and my partner claiming there is another sac next to the sac that was measured. Is it possible the doctor may have missed a possible 2nd sac?