The Oscar winning actress, 47, said when she hit rock bottom she was unable to wash or change her clothes.
Thompson told Easy Living magazine: "I've certainly been there, in various depressions, when you never wash, and wear the same things all the time.
"It's the sort of depression that doesn't necessarily make you want to kill yourself - you just don't want to be, you want to switch it off and stop.
"That's not the same as saying 'I'm going to kill myself'. But it's a feeling I know well."
Emma, who has a six-year-old daughter Gaia with her second husband Greg Wise, added: "The work I've done since Gaia was born has been quite intense at times, and it fed into a sort of postnatal depression that sent me right off the edge."
The Sense and Sensibility star went through four cycles of IVF, three of them after Gaia's birth, which contributed to her depression.
She told the magazine: "It was hellish after Gaia was born, trying to have another baby through IVF.
"That was terrible - I blamed myself, and no-one could persuade me that it wasn't my fault - and that led to another depression."
Thompson, currently playing a struggling author in Stranger Than Fiction, said: "I counted other people's children for years. But I'm fine about it now."