The pills, also known as vitamin B9, increase the chances of conception and help to nurture a healthy baby.
"Kate is taking supplements every day. Jamie suggested she take folic acid because of its benefits. Her pals have been teasing her that she's taking acid so openly - considering her reputation," thesun.co.uk quoted a source as saying.
"While there's been lots of speculation about Jamie and Kate getting married, what they want is to extend their family," added the source.
Moss already has seven-year-old daughter Lila with ex-beau Jefferson Winston Hack, editor of "Dazed & Confused" magazine.
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