Participants came from the Framingham Heart Study, a large study that has been going on since 1948 to look at heart disease risk factors in Framingham, Massachusetts.

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It is modeled on the famous Framingham Heart Study, which helped spot cholesterol and other risk factors for heart disease.

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This, the study's authors suggest, was not surprising, given that on the whole only women with few risk factors are likely to be able to book a homebirth and they were thus a self-selecting group.

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