Why is the threshold between human embryo and human fetus defined as 8 weeks after fertilization? What's happening on the 8th week?

Note: definition of embryo based on Wikipedia's current definition.

  • $\begingroup$ Are you seeking for a science article that define the words "baby" and "embryo"?! This is going to be hard to find as these words are so easy and common. Similarly there are probably no science article who define the word "increase" as everybody know what it means. But I don't know maybe someone will find what you're looking for. But you should probably not expect a different definition from the wikipedia ones! $\endgroup$
    – Remi.b
    Nov 6, 2014 at 21:02
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    $\begingroup$ So the question might be "Why did we use the threshold between the definitions of human embryo and human fetus at 8 weeks after fertilization and not at 7 weeks or 9? What's happening at the 8th week?". Is that right? $\endgroup$
    – Remi.b
    Nov 6, 2014 at 21:07
  • $\begingroup$ @Remi.b does my question now appear more clear? Are there any further specifications that I might make? $\endgroup$
    – J.Todd
    Nov 6, 2014 at 21:22
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    $\begingroup$ I would recommend you to just reduce your whole question to what you copy-pasted from my comment (and modify your title accordingly). $\endgroup$
    – Remi.b
    Nov 6, 2014 at 21:25
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    $\begingroup$ I delete my answer as it does no longer address your question. I don't have enough knowledge in developmental biology in order to answer your question, so I'll let someone else working on your question. You may still receive some comments on your introduction but I think the question will be accepted from now on. You should cite wikipedia for the definition you are using of embryo. $\endgroup$
    – Remi.b
    Nov 6, 2014 at 21:35

1 Answer 1


An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation. However, animals which develop in eggs outside the mother's body are usually referred to as embryos throughout development; e.g. one would refer to a chick embryo, not a "chick fetus" even at late stages.

I can answer what "more advanced" means with the following 2 pictures. I think you can see the difference. (Real photos were too disgusting for me.)

7th week embryo

  • Figure 1 - 7th week embryo - source

8th week fetus

  • Figure 2 - 8th week fetus - source

For the sake of blind people and google, the 8th week fetus has a face and a human form, while the 7th week embryo doesn't.

From the 10th week of gestation (8th week of development), the developing organism is called a fetus.

All major structures are already formed in the fetus, but they continue to grow and develop.

Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.

Because the precursors of the organs are now formed, the fetus is not as sensitive to damage from environmental exposure as the embryo was. Instead, toxic exposure often causes physiological abnormalities or minor congenital malformation.

So the scientific explanation of the threshold is that the precursors of all the major organs are created by that time.

  • $\begingroup$ I don't think this answers the question. That the embryo/fetus becomes more advanced over time is a given, but it does not answer the question of why the 8/9 week divide was chosen as a threshold. A 12 week embryo/fetus also has a more "advanced form" and is more "human-like" than a 11 week embryo/fetus. $\endgroup$ Nov 7, 2014 at 10:02
  • $\begingroup$ @fileunderwater Edited my post. $\endgroup$
    – inf3rno
    Nov 7, 2014 at 14:47

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