Is there a medicine to increase my chance of havings twins?

Be careful: What you wish for. While having twins might seem great, but remember they are at greater risk for premature birth and all the long term health risks that come with it. That being said many of the fertility treatment drugs like clomiphene and the various injectable gonadotropins, do increase your risk of multiplets(twins and greater). Again, think long and hard about why you want to have twins!

Yes.: A medication used to help infertile couples, called Clomid, (clomiphene) when given to a normal woman can increase her chance of having twins to about 33%. See your OB/Gyn for other, more natural ways to increase your chances of twinning, including dietary strategies. Yams, or cassava, for instance increase some people's chances of having a twin pregnancy fourfold. Eating dairy also increases your chances.