I have heard that there is a gemara that discusses what side a newborn is supposed to breastfeed from first.
If this is true, where is this gemara and what's the idea behind it?
The Kaf HaChaim writes in Yoreh Deah siman 116 number 149 "a woman that begins to nurse her son should begin nursing from the left breast first". He writes that his source is the Tzava'as Rebbe Yehudah HaChasid #69 and the sefer Shmiras HaNefesh #17.
The sefer Mishnas Yehoshua footnote #18 (on the linked page) mentions that according to the sefer Shmiras HaGuf Ve'haNefesh, a person does not have to be concerned to do this with a daughter (only with a son).