Final answer:
During the second trimester, an average-weight woman should increase her daily calorie intake by about 300 calories to provide for the growing fetus without risking excessive weight gain.
Step-by-step explanation:
An average-weight woman is advised to increase her calorie intake by approximately 300 calories per day during her second trimester to support the growing fetus. This is because the fetus is growing most rapidly during the last trimester, and the extra calories contribute to the necessary energy for the development of maternal tissues and the added work of the heart and respiration due to pregnancy. It is important to note that doubling caloric intake is a myth and can lead to excessive weight gain, which might be detrimental to both the mother and baby.