在收到一些刺激后，通过超声观察胎儿不再回应刺激然后刺激被接受为“安全”。

After receiving a number of stimuli, the fetus no longer responds to the stimulus as observed by ultrasonography and the stimulus is then accepted as "safe."

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值为- 1表示在收到的回应标头上没有任何限制，值为0则表示所有要求都会失败。

A value of -1 means no limit is imposed on the response headers received; a value of 0 means that all requests fail.

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