Statistically, the frequency of twin births varies by ethnicity. For us it was probably around 1/60, which theoretically might mean the odds are 1/3600 for people with two pregnancies. However, it's more complicated because women can be genetically predisposed toward releasing multiple eggs every ovulation cycle, so after you've had one set of twins, the odds of you having another are higher since there's a chance you are pre-disposed. All of that which is to say I have no idea what the odds are.