The girl, who was placed in the couple's family home, was taken to hospital in June, malnourished and with serious illness symptoms.
According to the prosecutor, a doctor's statement shows that the girl has suffered from so-called "refeeding syndrome".
- At that time she suffers from hunger.
When she then gets food, there is an imbalance in the body.
This leads to a life-threatening condition that requires care, says chamber prosecutor Moa Blomqvist to the newspaper.
The two foster parents, who at the time had custody of the girl, were arrested earlier this week after the doctor's statement about the girl's condition became clear.
They were arrested by Uppsala District Court on Friday, suspected of probable cause for causing other bodily injury.
They both deny any wrongdoing.
After the incident, the girl was relocated to another family home.
She is feeling well today according to the circumstances, according to the prosecutor.