Well, actually there is no tale at all, just simple explanation - stimulating nipples (like baby is feeding), it bring out hormone called oxcitocine (hope I spelled it right), and it couses uterus to squeeze (no, there was another word, sinonime, I forgot it - sorry,
English isnt my native
language). As Your baby hasnt born yet, it can couse
contractions, something like after having orgasm.