What factors play a role in concieving twins?

my fiance and i are expecting in december we've not yet had an ultra sound done, but both have a hunch there could be more than one baby in there, along with the hunch, odd occurance's such as randomly clicking a link that take's us to a pg for free stuff if you're expecting multiples, randomly ending up with 2 forks and a spoon or a fork and 2 spoons at dinner time, also seen 2 shooting stars 2 minute's apart, @ 14 weeks i look 6 months preg compared to pass pregnancies. my cousin has twin girls, but as far as i know those are the only twins in the family.