The Beic people worship a pantheistic religion that revolves around a feminine manifestation of divinity. The creator is a mother goddess that is imbued in all of creation, but resides in the Immaterium, a spiritual world parallel to this one. She is not distinct from reality, but is identical to it and all-encompassing. As she is a part of creation, all life is sacred, down to the lowliest creature as well as the animals they hunt.
The land That the Beic inhabit is a large and tropical continent with warm temperatures all year-round and much fertile soil. Society is organized into extended family groups called bands led by an elder woman and consisting of her descendants, plus males who have married into the family. Bands rather than marriages are the basic economic unit: members work for the band as a whole, and wealth is pooled. Raising children is a task of the entire band. The optimum size of a band is one to three dozen people; when the band becomes larger than this, it splits. When an elder dies and has two adult daughters, they each become the nucleus of their own bands. When a band splits, it will abandon its old fields and begin two new plots. Noble families own the land and do not split off, while other bands are given the right to live off of the estate.
Humans existed as separate physical entities, but maintain an attachment to the Immaterium due to their connection with god. When a child is ready to be born, it must pass through the spirit world into the physical. This other world is not completely benign and serves as both a light and dark reflection of the material universe, raw emotion given physical form. Malevolent spirits exist in this realm. Although rare, they can affect the fetus and have various effects. Physical and mental deformities can develop (tentacles, horns, extra eyes or limbs, etc).
While the creator encompasses humanity in its entirety and represents a balance of its emotions, each of these evil spirits represent an emotion taken to its extreme. Magical runes are placed on the mother to act as a barrier of protection to prevent this through a ritual, and must be reapplied periodically. This is performed by midwives, who study this craft during their training. While runes are an effective deterrent, they are not a foolproof method.
These children who survive birth are not born evil, but are treated with suspicion by the world due to their "impure" taint. In ancient Greece, infanticide was practiced with unwanted newborns. A mother would present it to her husband. If he accepted it, it would live. If not, it was customary to expose it to the elements so that it would die naturally. I would like to represent various cultures in this world as responding to these events differently. Would a matriarchy be more likely to kill "defective" infants more so than our own past cultures? How could they rationalize it?