(NEW YORK) -- The still-slender but growing Duchess of Cambridge is having some cravings, Us Weekly reports.

“She’s got sugar on the brain,” a source close to the mom-to-be told the magazine.  “Chocolates, biscuits, cake.”

When Kate Middleton pops into the nearest Starbucks for a cup of decaf, the source said she sometimes picks up a sweet treat too.

No one really knows why pregnancy cravings occur, though theories abound that they are the body and baby’s way of asking for a nutrient the mother is lacking.  Whatever the reason, the majority of moms-to-be have them.  One South African study reported that up to 84 percent of pregnant women experience some form of food craving.

According to the study, cravings tend to fall into three groups -- sweet, sour or savory -- with a yen for sweets by far the most common.  

A few pregnant women seek out strange snacks such as chalk, clay or dirt.  The urge for odd edibles is called pica, after the Latin word for magpie, a bird notorious for ingesting almost anything.  According to the American Pregnancy Association, pica cravings may be the result of an iron deficiency.

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