What are the main differences between singleton pregnancies and twin pregnancies?

Twice as many babies. Because the number of babies is doubled, the uterus grows larger faster; this will typically lead to a premature delivery at <37 weeks. There are also increased risks of pregnancy complications such as pre-eclampsia. Delivery route can also be affected if the first twin is not head down. If the first baby is head down, then a vaginal delivery of both twins can usually be performed.
High risk. The multiple pregnancies always considered to be the high-risk pregnancies, since they do have much more complications.