Pregnancy and Eating Disorders

I've been thinking quite a bit about the effect of pregnancy on recovery from an eating disorder. I have had clients whose resolve to feed themselves adequately, or stop purging, increases dramatically once the pregnancy is discovered, as would seem to make sense. But I am currently dealing with a young woman whose symptoms have become much worse; she has just been hospitalized in order to stabilize her medical situation, and hopefully, help her gain strength and motivation to continue caring for herself on an outpatient basis. So why is pregnancy a positive motivation for some and a significant stress for others?