How do woman with eating disorders get through pregnancies?

so on the show whats eating you there were anerexic women with children and they had the disorder before the kids. if they were having so much trouble quitting on the show howd they eat enough to get through a pregnancy? wouldnt the baby die from malnutrition in the womb? just wondering how they and other woman did it. i dont have an eating disorder so theres no need to say how bad they are cuz everyone already knows that. if youve gone through it or someone you know has id like to know cuz i just dont get it