science time : https://en.wikipedia.org/wiki/Lactation
It is also possible to induce lactation without pregnancy. Protocols for inducing lactation are called the Goldfarb protocols.
Using birth control pills to mimic the hormone levels of pregnancy, then discontinuing the birth control, followed by use of a double electric breast pump for 15 minute sessions at regular 2-3 hour intervals (100+ minutes total per day) helps to induce milk production.
In humans, induced lactation and relactation have been observed frequently in some cultures, and demonstrated with varying success in adoptive mothers.
It appears plausible that the possibility of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage, especially in groups with high maternal mortality and tight social bonds.
The phenomenon has been also observed in most primates, in some lemurs, and in dwarf mongooses.
Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods.
Domperidone is a drug that can induce lactation.