The cluster of developing cells from conception until birth is called an embryo.

Here's a breakdown:

* Conception: The moment a sperm fertilizes an egg.

* Zygote: The single cell formed by the union of sperm and egg.

* Embryo: The developing organism from conception until approximately the 8th week of pregnancy.

* Fetus: The developing organism from the 9th week of pregnancy until birth.

So, while the term "embryo" encompasses the early stages of development, it's important to note that the terms "zygote" and "fetus" are also used to describe specific stages within the overall development process.