The table shows the weight of a newborn infant for its first year. During which three-month period was the infant's weight gain the greatest?

0 Months: 7 1/4 lb

3 Months: 12 1/2 lb

6 Months: 16 5/8 lb

9 Months: 19 4/5 lb

12 Months: 23 3/20 lb