Awhile ago, someone on Lactnet questioned how hand expression works when 
the recent ultrasound research has suggested that negative pressure is 
the most efficient way to get milk out of the breast.

It has been believed for quite some time that babies modulate positive 
and negative pressure to remove milk from the breast. It would be 
impossible for infants with cleft palates to get any milk from the 
breast if negative pressure were the only possible player. (Some 
cleft-affected infants indeed don't transfer milk, but some do transfer 
at least some).

Manual expression uses positive pressure to increase the pressure inside 
the breast, and make milk flow outward in an attempt to equalize 
pressure inside and outside.
Catherine Watson Genna, IBCLC  NYC

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