In almost all mammals, lactation, or more correctly the suckling stimulus, induces a period of infertility, usually by the suppression of ovulation, which serves to provide the optimal birth spacing for survival of the offspring. McNeilly, A. S. 1997. Lactation and fertility.
There is thought to be little or no difference in milk composition whether lactation is artificially induced or a result of pregnancy. Wilson-Clay, Barbara (1996).
Source: Wikipedia > Lactation
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