The boundaries, were proposed in the mid-1980s by two scientists, Garrow and Webster, and just kind of stuck.

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"They are to some extent based on health premises as understood by Garrow and Webster in 1985 when the world looked rather different, but also they are a statistical construct, " says Prof Cole.

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"Much of the controversy about abortion is really stimulated by the interest groups on both sides of the political question, rather than by ordinary Americans, " said David Garrow, a law professor at the University of Pittsburgh, and a longtime Supreme Court scholar.

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