The birth of twins is a calamity of the first magnitude, and spells disaster for them and the unfortunate mother.
The underlying idea is that it has been ordained that mankind should propagate his species by single births, in contradistinction to animals ; for a woman to bear more than one child at a birth is to degrade humanity to the level of the brute creation.
Plural offspring is nature's law for goats and dogs ; for a woman to imitate them in this respect fills the Ibo with unspeakable disgust. Mother and children are cursed and subjected to contempt and ill-treatment.
The woman makes no attempt to defend herself or her children ; she accepts the situation dumbly, and merely wonders why she, especially, has been selected for such dishonour. Her hatred of her offspring is as bitter as that of her relatives and neighbours.
She turns from them with loathing and despair and, unless compelled, will make no effort to nurse them. It is believed that in some mysterious manner there has been an unholy alliance with an evil spirit during sleep, and the second child is the result. With all haste the abominations must be removed.
The children are thrust into an old waterpot without even a passing thought for the pain inflicted. Cocoanut fibre or leaves are thrown in to cover them, and the pot is then deposited in some lonely spot in the bush. The newly born infants receive no attention whatsoever. They are cast away at once, as unclean in the sight of gods and men.
It is astonishing how long some of the children survive after being treated thus ; this extraordinary vitality being due, perhaps, to the fact that the umbilical cord is not severed. In one case of castaways with which I had to deal, they had been lying a whole night, and the greater part of the next day, in the bush — at least sixteen hours.
One died from injuries either deliberately inflicted or received when being forced through the neck of the pot ; the other, and uppermost, child had escaped serious injury and has since grown into a sturdy boy, mother and child having been kept under supervision for some months at the medical mission.
This method of dealing with twins is reckoned to be the only one open to the parents, as by acting thus they hope to avert further calamity.
To permit the children to live would amount to a direct challenge to the malignant spirits, and the parents, and indeed the whole community, would be exposed to all manner of dangers.
There is no alternative for the Ibo but to cast them out.
Among the Ibos of Nigeria, an account of the curious & interesting habits, customs, & beliefs of a little known African people by one who has for many years lived amongst them on close & intimate terms
by Basden, George Thomas
Published 1921

source: http://www.history.com.ng/66/the-fate-of-twins-in-pre-colonial-nigeria