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母亲与婴儿的血液在胎盘发生交汇，氧气和营养物质通过脐带进入婴儿体内，而代谢废物由母体排出。出生时，婴儿带着胎盘和脐带一起娩出，这时脐带已经完成它的历史使命，产科医生或者助产士把脐带剪断、结扎、消毒，脐带残端形成的“遗迹”，就是肚脐。
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