One of Seiga's spell cards is Yǎng Xiǎoguǐ – literally "Raise Little Demon". And yes, they're dead babies. You know, lucky charms and all that.
There is also an angle that these spirits chased after Reimu/Marisa/Sanae/Yōmu because since they're miscarried fetuses, they yearn to go back to the womb.
