My first question is are you sure you are pregnent? Just because you miss a cycle or two does not necessarily mean you are with child. Thew weight gain may be a matter of biochemical changes in the endrocrine system. Remember that "the pill" contains chemicals that are now absent from your body. Typical physiological responce to this condition is rapid weight gain. Plus you have crested the threshold for natural body adjustments. The human body operates in stages. Each stage is denoted by some physiological change, weight gain, poor vision or hearing, loss of appitite, loss of motivation, and so on. This is the bodys production method of adjusting to new environments and conditions. You say your 25. You are fully out of your primary physical and psycholgical development stages. Your body is setting you up for the long run between this stage and the breakdown stages. The missed periods could simply be a matter of stressors in your life that were not previously present, like thinking about the possability of having twins for one.
If you are pregnent(congratulations I hope), then your probability is definatly increased for having twins. This may or may not be the result of you stopping birth control. You are genetically marked as being predispositioned for twins. I do not know the statistical data about the correlation between ending chemical birth control and the increase of birthing twins. I do know this would be a very difficult thing to prove or disprove. Simple logic would tell me that this probably unlikly. As we all know twins develop at the cellular stage. Either two eggs are fertilized or a single egg splits. Becuase of this fact, this process is considered as an "Inherent Trait". Because you are predispositioned for twins (another inherent trait) if you do birth twins I would be more inclined to rely on that rather then the chemical changes in your body which would suggest that your genetic encoding has been altered. I base this theory on the twin birth rates before and after the inception of chemical birth control methods. The rate has changed very little over the span of human existance (or at least as long as records of such things have been kept). So long story short if you have twins blame your mom or your husbands/significant others mom lol.