10. Pumping and squeezing breast milk on Shabbos:[1]

Into a cup and the like: A woman may not squeeze milk from her breasts into a cup or into a pot in order to nurse [the milk] to her child. [See Q&A regarding if the child refuses to nurse and there is danger involved]

The reason for this is: because one who milks into a vessel is completely detaching and is liable for the threshing prohibition, and it was only permitted to do a Biblical prohibition in a case of life threatening danger.

Trickling milk in order to stimulate her child to nurse:  However it is permitted for a woman to squeeze out some milk [and have it trickle down her skin and the like[2]] in order to stimulate her child to take hold of her breast and nurse. [However she may not squeeze the milk into his mouth in order to stimulate him –See Q&A below]

The reason for this is because:  as since this milk [that is squeezed] is going to waste it does not contain a Biblical “detaching” prohibition, but rather a Rabbinical prohibition as explained in chapter 320 [Halacha  21] and for the need of a child [the Sages] did not apply their decree.

Squirting milk onto one who is spellbound: However it is forbidden to squirt from her milk onto one who has been overtaken by an evil spell, as he is not in any danger and is not in great pain for us to rescind for him a Rabbinical prohibition to be done through a Jew, as [opposed to] what was permitted by one who has heart ache and the like. See chapter 330 [Halacha 9].    

 Is it permitted for a woman to squeeze out milk from her breast to relieve pain?[3]Even a child which is for certain an eighth month [child], which is [therefore] forbidden to move according to all, his mother may bend over him and nurse him due to the pain of the milk which pains her.

Similarly she herself may remove with her hands the milk which is causing her pain. [See Q&A regarding using a pump]

The reason for this allowance is: as since this milk is going to waste it does not contain the [Biblical] detaching prohibition but rather [only] the Rabbinical, and due to her pain they did not decree [against removing it].

Summary-Pumping milk from a woman’s breast on Shabbos:
Is forbidden to be done into a vessel due to the Biblical detaching prohibition.  However it is permitted for a woman to squeeze out some milk and have it trickle down her skin in order to stimulate her child to take hold of her breast and nurse. As well it is permitted for her to squeeze milk from her breast onto the ground in order to relieve breast pain caused by the milk.

Q&A

May a woman pump milk out to feed her baby if he refuses to suck from the breast?[4]
If there is nothing else available for the child to eat [such as formula] and there is thus possible danger involved then she may pump out less than a Grogeres[5] worth of milk at a time and feed him it, and then pump out another Grogeres worth and feed him it. She may not pump out more milk until the child is fed the previous milk that was pumped.

May one squeeze the milk into the baby’s mouth to stimulate it to eat?[6]
No! One may not squirt the milk into the mouth of the baby as in such a case it is not going to waste and thus squeezing it contains a Biblical prohibition. [See footnote for opinion of M”B[7]]

May a woman use a pump to release milk in order to relieve breast pain?[8]
If plans to store this milk: If the pump contains a bottle which will store the milk then it is forbidden to be done if she plans to use the milk.

If plans to throw out the milk: If she plans to throw out the milk from the bottle some Poskim[9] rule it is nevertheless forbidden. Others[10] however rule that this is allowed to be done. The Ketzos Hashulchan[11] rules that one may be lenient like these opinions to pump out less than a Grogeres[12] worth of milk at a time and then spill it out prior to pumping again.[13]

If the milk goes directly to waste from the pump: It is allowed according to all opinions to use a pump to release the milk, in order to relieve pain, if the pump squeezes out the milk directly onto the floor or garbage.