Writing on her blog, the 30-year-old Brazilian who gave birth in December, said she "wasn't here to judge" other mothers.
She had said feeding her son Benjamin had helped her lose her pregnancy weight.
Department of Health figures show only 20% of mothers are still breastfeeding when their babies are six months old.
Initially, 80% of new mums attempt to breastfeed.
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Bundchen made her initial comments supporting long-term breastfeeding in an interview with Harper's Bazaar magazine.
"I think there should be a worldwide law, in my opinion, that mothers should breastfeed their babies for six months," she said.
"Some people here [in the US] think they don't have to breastfeed, and I think 'Are you going to give chemical food to your child when they are so little?' "
'Constant search'
But in a subsequent entry on her blog, she said: "My intention in making a comment about the importance of breastfeeding has nothing to do with the law. It comes from my passion and beliefs about children.
"Becoming a new mom has brought a lot of questions, I feel like I am in a constant search for answers on what might be the best for my child."
The world's highest paid supermodel, who is married to American football star Tom Brady, had a natural birth at her home in Boston after meditating throughout her eight-hour labour.
Television star Denise van Outen recently said she gave up breastfeeding her daughter Betsy after less than a month because she didn't want photographers taking pictures.
"I probably should have persevered a bit longer than three weeks," she said recently. "But I can't be sitting in Starbucks and breastfeeding, because they [photographers] are taking pictures."
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