What are they?
Well, heredity is an influencer. If you're a twin or if twins run in your family, you're slightly more likely to have a set yourself. Women who are fraternal twins have a one in 60 chance of bearing twins.
Your race can also predispose you to a higher chance of having twins. Twin are more common than average in African Americans and less common in Hispanics and Asians.
Lastly, your history of having twins influences you twin-pregnancy probability. Once you have a set of fraternal twins, you're twice as likely to have another set in future pregnancies. If you are a multiple egg-dropper once, you are likely to be a multiple egg-dropper again. :):):)
So there are some controllable influencers and there are some "what-you-are-dealt-in-life" influencers behind whether you are more likely to have twins or not.
I think most of us just shoot for happy and healthy pregnancies and babies but this is certainly some good information to consider if you are hoping to conceive multiples.
Source of facts from the great site Baby Center.com -