Nature's say

In order for their children to become the strongeset possible, the orcs reason that they must remove the weak before they are even born. Thus, they create a poison. If the fetus dies they think it a shame, but it just wasn't strong enough. The women drink a mixture of poisonous roots and berries to drink, and if the child survives it is deemed worthy of being born an orc. This way, they reason, only strongest children will be born and carry their race forward into the world.

A mother can refuse to drink the mixture, but the child will have to work much harder than its peers to prove its strength, and may not even then gain enough respect. The other orcs argue that they can't know for sure if they are worthy of being born an orc since its mother didn't take the mixture. You can't know for sure Nature said they would be worthy. Although such views are not too common, since they proved their strength, and it is by an orcs strength that their worth is measured.