noun
littermate; litter (of animals)
Refers to animals born from the same mother at the same time. Can denote an individual littermate or the group as a litter.
この子犬たちは同腹子です。
These puppies are littermates.
同腹子の中で一番活発な子猫を選んだ。
I chose the most active kitten among the littermates.
Compound of 同腹 (same belly) + 子 (child). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.