I had an ultrasound today. my gestational sac measures 16mm. she said there was a yolk sac but no fetal pole yet. does this mean miscarriage?

No confirmed IUP: How many weeks pregnant are you? If you are very early, it may mean that they identified a gestational sac, but it may be too early to see a yolk sac, which would confirm an Intrauterine Pregnancy (IUP). Did they tell you to come back for another ultrasound in 1-2 weeks?

No: if there has been no bleeding it may be to early to show the fetal pole and an ultrasound in a couple weeks should show more good luck