Formal/Scientific:
* Fetus: This is the most accurate term for an unborn animal in the later stages of development.
* Embryo: This term is used for an unborn animal in the earlier stages of development.
Informal:
* Unborn calf: This is a simple and straightforward way to describe a calf in gestation.
* Calf in the womb: This is another straightforward option.
Other:
* Calf in the making: This is a more poetic and informal way to refer to a calf in gestation.
The best term to use will depend on the specific context and the desired level of formality.