When you get a vaginal ultrasound it should be able to detect two fetuses. If you hear the baby's heartbeat before you do an ultrasound the doctor can sometimes detect both heart beats and then have to do an ultrasound to see if there are multiples.
usually it can be determined correctly through ultrasound scanning. family history with multiples also plays an important role in determining this factor.
An ultrasound will obviously be able to detect if their are twins, but also, you will probably be very sick, very hungry (eating for three! ), and very large in the stomach!
