The mishna says that if the mother gives "birth" to a lump of blood the next child is not a bechor. The gemorah says that it's not tumah for a nivaila sice it is butel b'rov in the blood.
The steipler asks why then is the mother concidered to give birth to child, all the remains of the child should be considered non existance, since it is butel b'rov-(see over there what he answered)
In my humble opinion, there are some actions that depend on the object your doing it to. For example, you can't eat without food, you can't swim without water and etc. Even if something is "batel", since physicaly the object is in existance, you can do an action to the object that will define that action. So, even though the child that was born does not halachicly exist, but since phisicly it exist, the mother can be considered giving birth (which is defined as to a child)
If someone would mix on shabbos food that was not cooked into cooked food and put it on the fire , of course he'll be chayiv for m'vashel, even though the non cooked item was butel, still you are still cooking

This message has been edited. Last edited by: Rav Chaim,