It tends to be transmitted maternally, where you would have the likelihood of releasing two eggs. I would like to know a little more about the family history. Yes, they are identical. Conjoined twins are monozygotic. They do have to be identical.
Conjoined twins are very rare.
The occurrence rate is one in 500 twin pregnancies and one in 80,000 births. My ultrasound shows that my baby is very small. One thing it might mean is that your dates might be an error. One thing to look at is whether you calculated your menstrual history correctly. If the baby is small, it needs to be watched carefully to make sure of adequate growth.