Similar instances, which are well known to students of teratology, are the Hungarian sisters (Helen and Judith), the North Carolina twins (Millie and Christine), and the Bohemian twins (Rosalie and Josepha Blazek).

The interspace between the thoraces may, however, have simply been the addition of the first artist who portrayed the Maids (from imagination ?); then it may be surmised that they were ectopagous twins.
"Pygopagous twins are fetuses united together in the region of the nates and having each its own pelvis.

In the recorded cases the union has been usually between the sacra and coccyges, and has been either osseous or (more rarely) ligamentous.

Sometimes the point of junction was the middle line posteriorly, at other times it was rather a posterolateral union; and it is probable that in the Biddenden Maids it was of the latter kind; and it is likely, from the proposal made to separate the sisters after the death of one, that it was ligamentous in nature.
"If it be granted that the Biddenden Maids were pygopagous twins, a study of the histories of other recorded cases of this monstrosity serves to demonstrate many common characters.

Thus, of the 8 cases which Taruffi has collected, in 7 the twins were female; and if to these we add the sisters Rosalie and Josepha Blazek and the Maids, we have 10 cases, of which 9 were girls.