By Dr Alan Lindemann
The American College of Obstetricians and Gynecologists (ACOG) recommends a woman's weight gain in pregnancy be relative to her pre-pregnancy body mass index (BMI). A woman who began pregnancy underweight, with a BMI of less than 18.5 (about 108 pounds for a 5'4" woman) will be advised to gain 28 to 40 pounds. A woman weighing what is considered normal (roughly 108 to 145 pounds for a 5'4" woman) is told to gain 25 to 35. A woman considered to be overweight (146 to 174 pounds for a 5'4" woman) is told to gain 15 to 25. And obese woman (175 pounds and up for a 5'4" woman) is told to gain only 11 to 20 pounds.
The implicit assumption behind these recommended weight gains depending upon pre-pregnancy weight is that the heavier woman can supply more of pregnancy's caloric load from her existing fat reserves and therefore needs to gain less during her pregnancy. These recommendations appear to be based upon the baby's requirements for nutrition. They are not.
The fetus growing in a 250-pound (113 kg) woman has the same nutritional needs as the fetus growing in a 115-pound (52 kg) woman. Fetal requirements are absolute, not relative to maternal size. The baby builds itself from the same materials, on the same schedule, in the same quantities, regardless of who is carrying it. These weight guidelines obscure the fact that the baby's requirement for growth and development depends upon what the mother is eating so the baby gets the nutrients it needs. The central question is what the mother is eating and whether the baby is getting what the baby needs. It is not whether the mother should gain a prederermined amount of weight based upon her pre-pregnancy size.
Stored fat lacks the building blocks necessary for developing fetal tissue. Stored fat contains no protein. No choline. No iron. No folate. A woman who restricts food intake to meet a constrained weight-gain target can deliver calories from her own fat, but she cannot deliver the micronutrients the fetus actually needs to develop. The baby, over the long run, is short-changed in ways that are difficult to measure. Calories and nutrition are not the same thing, but these pregnancy weight guidelines treat them as if they are.
A doctor guiding an obese woman through pregnancy should not aim primarily at restricting her weight gain but rather should aim at supplying the baby's nutritional needs in full, while monitoring maternal blood pressure, blood sugar, and fetal growth attentively. The eating pattern that achieves this is the same pattern that serves any pregnant woman: protein, fat and carbs with every meal, three large and four small frequent meals (snacks) through the day, eggs, fish, real food, no carbohydrate-only meals. The patient's number on the scale is not the target. The nutritional adequacy for the baby is the target.
