if you've naturally conceived fraternal twins, your chances of naturally conceiving fraternal twins again is said to be 1 in 4. It is assumed that you have a tendency to ovulate multiple eggs. This is why fraternal twins are hereditary - because a tendency to ovulate multiple eggs at once is genetic.
It is different for identical twins though. Chances of spontaneously conceiving a second set of identical twins is 1 in 70 000. although it does happen, of course!
:yeahthat: but I do know of a lady that had identical twin boys AND identical twin girls in 2 separate pregnancies. I also know of a set of identical twin girls who have both had identical twins themselves (one a set of boys and the other a set of girls). Knowing the odds of having identical twins in the 1st place I thought that both of these cases were pretty special considering the identical twin thing doesn't 'really' run in the family like the old wives tale would have you believe.