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Fetus is the usual American spelling of . It names a baby developing in the womb, after the very early embryo stage and before birth. In scientific writing, even in Britain, fetus is now very common; still turns up in older British books. It is a medical, biological word — you will see it in textbooks and in news about pregnancy, not in baby-talk. People sometimes use it for any baby; strictly, doctors use embryo for the first weeks and fetus later on. Same word, two spellings; not two different things.
胎兒（美式拼法；英式舊拼 foetus。台灣醫學英文也常用 fetus）
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
fetusnoun/ˈfiːtəs/
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An unborn or unhatched vertebrate showing signs of the mature animal.
- Several feti were removed from every rats' uterus, stripped of their membranes and allowed to lie in the peritoneal cavity connected to the placenta by the umbilical cord and with the placenta still attached to the uterine wall.
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A human embryo after the eighth week of gestation.
- The sequence is: molecules in reproductive systems, then gametes, zygotes, morulas, blastocysts, and then fetuses.
- Though scientists do not know how stress affects gestation, Fukuda theorizes that the vulnerability of Y-bearing sperm cells, male embryos and/or male fetuses to stress is why “subtle significant changes in sex ratios” occur. […] The factors that filter out who “gets through” from conception to birth include chromosomal or genetic abnormalities of the fetus or the mother’s stress response to changes in her environment, Catalano said.
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archaicA neonate.
- The real essence of that or any other sort of substances, it is evident, we know not; and therefore are so undetermined in our nominal essences, which we make ourselves, that, if several men were to be asked concerning some oddly-shaped fœtus, as soon as born, whether it were a man or no, it is past doubt one should meet with different answers.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。