This has led steinman to suggest the introduction of growth - hormone treatment for cows in the 1990s , to enhance milk and beef production , may have contributed to a rise in the number of twins over the last decade 然而，生育双胞胎的比率在上世纪90年代仍持续增长则可能是由于人们为了给奶牛催奶或增加牛肉产量使用生长激素的结果。
