Scientist: Babies cry at night to prevent conception of additional sibling

a. A parent, or; b. Familiar with pre-modern society

Most children once they have had their Maslow Hierarchy met will go back to sleep quite happily.

Furthermore, prior to 20th Century/Western-style parenting the raising of a child was diffused amongst the community including multiple wet-nurses.

The idea of a maternal fatigue is a product of modern society which has removed the community aspect from child-rearing adding to the professional, parenting and lifestyle burden of a mother/father.

[Source: 3 children anecdotal evidence (which counts for more than a highly speculative hypothesis without quantative/qualitative analysis in my mind).]

EDIT TO ADD If this study is to be believed then it flies in the face of evolutionary principles. A baby who screamed to prevent sibling rivalry would be placing the entire family, including themselves, within danger of predators.

The perceived long-term benefits from an evolutionary standpoint would be outweighed by the short term negative impact on the security and unity required for a functioning family unit whose primary purpose is to raise a child to an independent age.