A mother who is completely and utterly stoned with not be able to make up the bottles properly. It is quite likely she will not give a shit whether the bottles are sterilised or whether she has the right ratio of powder to water. Can you imagine that someone who is completely stoned will carefully make up the bottles with cooled boiled water and throw way any unused milk after an hour. Would someone who is out of their head on cannabis bother to check the used by date on formula?
Many habitual drug users cannot be arsed to do very much. Keeping a kitchen clean will not be high up the priority list.
What would be interesting would be to know how much cannabis actually gets into breastmilk. Have the prosecution actually analysised a sample of her breastmilk to know if a measurable of amount of canabiods actually exists in her milk?
This link shows that very small amount of cannabis gets into breastmilk.
www.cannabis-med.org/english/faq/20-breastmilk.htm
It is hard to know whether delays in motor skills are caused by poor parenting or by the cannabis in the breastmilk. A monkey is completley stoned won't have the energy to chase after its offspring. The truth of the matter is that no baby should be looked after by someone who regularly takes non prescribed drugs. Being a cannabis user and a good mother are completley incompatible.
My guess is that the poor baby probably inhaled a lot of cannabis passively.