Article in English: People low on nutrition are less likely to have twins.  All vitamins are good, but folic acid supplements are proven to increase your chances.  You can get them at any pharmacy.     Folic acid is now recommended for all pregnant women as well; it prevents birth defects.  However, you don't want to take more than 1000mg a day. Overall, underweight people are less likely to have twins.  Overall, being better nourished or overweight may increase your chances of twinning.  Better nourished means to put on weight in a healthy manner.  Consult your doctor about any plans for weight gain. There are foods that are possibly linked to better chances of twins.  A study conducted by a leading fertility specialist found that women who consume dairy products during the timeframe they're trying to conceive increase their chances of having twins by as much as 5 times over women who avoid such foods.  Insulin-like Growth Factor (IGF), which is produced in the livers of cows, is believed to be the chemical instigator in this phenomenon. Others suggest drinking milk of cows treated by rBGH hormones can influence human women to have twins more frequently.    An African tribe whose diet is rich in wild yams (cassava) boasts a twin birth rate 4 times higher than the global average. Nutrients in the vegetable are believed to stimulate the ovaries into producing more than 1 egg during ovulation.Many doctors are skeptical that the yams have anything to do with twins.  On the other hand, there is no harm to eating yams, and they are delicious. Try stopping the pill right before trying to get pregnant.  When women first stop taking the pill, their bodies work furiously to re-regulate their hormones. During the first month or two after getting off the pill, ramped up ovaries sometimes release 2 eggs. This is not proven either, but again can't hurt.  Some studies suggest it is true.

Summary in Vietnamese:
Bổ sung vitamin. Có chế độ ăn uống giàu dinh dưỡng và ăn các loại thực phẩm nhất định. Dùng những thực phẩm làm từ sữa và củ sắn. Ngừng sử dụng thuốc tránh thai.