Calcium is one of the most important elements in our bodies. It is needed to manufacture bone and is essential for milk production. Toy breeds such as Yorkshire Terriers, Maltese and Chihuahuas do not have the reserves to handle calcium demands for a large litter of puppies.
During normal times, a dogs dietary calcium needs are met from dietary needs. When a toy breed becomes pregnant and fetal skeletons start to develop, a strain is put on maternal calcium levels. When she starts to lactate (produce milk) that additional strain on calcium stores bring her close to "the edge". Her body can not keep up with calcium demands.
Eclampsia can occur anytime before the puppies are whelped up to about a month after delivery. The pups are not effected since their needs are being taken care of by the mother.
A drop in serum calcium is the result which leads to all associated clinical signs in the female animal.