halacha - Why doesn't a ben niddah acquire the status of a mamzer?

One who commits incest is punished with Kares (spiritual separation from the divine source), and any product of incest has the status of a Mamzer.

One who has relations with a Niddah is also punished with Kares, yet the product of such a union does not acquire the status of a Mamzer.

Shouldn't the product of both unions have the same status?

Answer

The Gemara says in Kiddushin that it's derived from a verse "ותהי נדתה עליו" - that even when one is a Nidda, there still is "Haviya" (marriage). Therefore, Kiddushin by a nidda works.

If so, there are no issues of Mamzeirus.