The Prime Minister's wife shared her experiences with mothers at a children's hospice in London.
She told Risa Balynas – whose five-old-year Miles requires a lot of care – about Ivan who died aged six in 2009, after spending his life dealing with cerebral palsy and epilepsy.
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The mother of two from the eastern borough of Tower Hamlets said: "It was lovely to meet her. She was saying that Miles' condition was similar to her son Ivan's.
"I have a lot of pains and aches from holding him all the time. She said she had that with Ivan when he was younger.
"She told me she had a painful shoulder from holding him because he screamed and went into spasm.
"She understood how it affects you, physically as well as emotionally."
Samantha was visiting Richard House Children's Hospice in Newham, east London to promote Carers' Week highlighting the needs of Britain's six million unpaid workers.
She took part in an arts and crafts class at the facility, which supports 290 families and 192 children.
The Camerons have three other children - Nancy, eight, Elwen, six, and Florence, who was born in 2010 after Ivan's death.
But recently Samantha's husband David spoke of the loss of the big brother their baby will never know.
"I miss my Ive every day, but at least I do sometimes now feel happy and not sad when I think of him."