It's mostly one of the following that determines it:
Use of fertility drugs or going through fertility treatments such as IVF
Genetics (twinning runs in families)
Luck or a combination of factors that raise your chance of multiples like being somewhat overweight or over 35. I think there may also be a higher incidence of twins among mothers who have already given birth to several children.
Good luck,
Rachel
(mom of twins & of two babies who are a year apart)
