Cats who are pregnant or nursing a litter don’t increase the amount of carbohydrate in their diet by choice. Instead, the additional calories they select come from fat.

In a study presented last month at the Waltham International Nutritional Sciences Symposium in Portland, OR, researchers found that pregnant and lactating cats will, when given the choice, opt for increased fat rather than increased carbohydrate in their diets:

In the study, seventeen adult female cats were offered a choice of three
nutritionally-complete wet foods with different proportions of protein,
fat and carbohydrate. During pregnancy, the cats significantly
increased their total energy intake, and the amounts of protein and fat
consumed also increased linearly. When lactating, the cats with large
litters of four or five kittens further increased their energy intake,
consuming a significantly higher proportion of energy from fat and
reducing the proportion of energy from protein and carbohydrate. Total
fat intake tripled for the cats feeding large litters, and doubled for
cats with smaller litters of one to three kittens.

Previous research has shown that non-reproducing adult cats with normal
energy requirements have a limit to the amount of carbohydrate they will
consume in a day (Hewson-Hughes et al. 2011). Specifically, cats’
“carbohydrate ceiling” was found to be approximately 20g of carbohydrate
per day. The present study expanded on these findings and showed that,
while cats increased their intake of protein and fat during pregnancy
and lactation, their carbohydrate intake did not exceed this limit of
20g per day. The research therefore shows that cats’ “carbohydrate
ceiling” still applies during the increased physiological demands of
gestation and lactation.

Read more here.