According to the passage Eve concieved and bore Cain then she again bore Abel.
Genesis 4:1 - 4
ESV - 1 Now Adam knew Eve his wife, and she conceived and bore Cain, saying, "I have gotten a man with the help of the Lord. 2 And again, she bore his brother Abel. Now Abel was a keeper of sheep, and Cain a worker of the ground.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I think it is possible for two to be born, and not be twins.
It doesn't say in the scriptures, so we don't know.
Has anyone looked to see what the word "again" in the Hebrew is? It looks to me like it is the English word "continued." If a woman has a child and then continues in labor, I think that would make them twins.