【上位】 a very young child (birth to 1 year) who has not yet begun to walk or talk; "the baby began to cry again"; "she held the baby in her arms"; "it sounds simple, but when you have your own baby it is all so different" 非常に年の若く（生後1年までの）、まだ歩いたり話したりできない子供。
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【下位】 an infant born weighing less than 5.5 pounds (2500 grams) regardless of gestational age; "a low-birth-weight infant is at risk for developing lack of oxygen during labor" 妊娠期間にかかわらず、5.5ポンド（2500グラム）未満の重さで生まれた乳児。
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【下位】 infant born after 42 weeks of gestation; usually shows signs of placental insufficiency 42週の妊娠の後で生まれた乳児。通常、胎盤機能不全の兆候を示している。
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【下位】 an infant that is born prior to 37 weeks of gestation 妊娠の37週間前に生まれた乳児。
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