答：自世卫组织20世纪70年代末至儿童生长标准制定之前，世卫组织一直建议使用美国国家健康统计中心的生长参考。

A: Before the Child growth Standards were developed, WHO had been recommending the use of the us National Center for Health Statistics growth references since the late 1970s.

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来自国家健康统计中心的这些数据涵盖了美国人口86%。

The data are from the National Center for Health Statistics and cover approximately 86 percent of the U. S. population.

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