Final answer:
Women with a prepregnancy BMI of less than 18.5 should aim to gain 28 to 40 pounds during pregnancy. No additional calorie intake is needed in the first trimester, but during the second and third trimesters, an additional 300 calories per day should support the growing fetus.
Step-by-step explanation:
Women who are underweight prepregnancy with a body mass index (BMI) less than 18.5 are typically encouraged to gain 28 to 40 pounds during pregnancy. This recommended weight gain is based on the health and nutritional needs for both the mother and the developing fetus. During the first trimester, additional calorie intake is generally not necessary, although a weight gain of about 0.45 kg (1 lb) per month is common.
However, during the second and third trimesters, the mother's calorie needs increase by about 300 calories per day to support the growing fetus. Most women will gain about 0.45 kg (1 lb) per week during these later stages of pregnancy. It's important to note that a proper balance in weight gain is crucial, as excessive weight gain can be detrimental to both the baby's and mother's health, potentially leading to conditions like diabetes and obesity later in the baby's life.