General:
* Fetus: This is often used interchangeably with embryo, but it specifically refers to a developing organism after the initial embryonic stages.
* Conceptus: This is a more technical term referring to the developing organism from the time of fertilization until birth.
* Germ: A general term for the earliest stages of development.
Specific:
* Blastocyst: A specific stage of embryonic development in mammals.
* Zygote: A fertilized egg cell, the very first stage of development.
* Gestation: Refers to the entire period of development within the womb, encompassing both the embryonic and fetal stages.
The most appropriate synonym will depend on the specific context and the stage of development being discussed.