The imagery of the unborn is tied to the theme of sutures. Jackson notes that skulls show seams where the different bone parts were once separate and flexible. Fetuses must have flexible skulls so that they can squeeze through the birth canal. Perhaps Jackson implies that genesis is nothing but an artificial piecing together of parts. An embryo is what exists between two people: a link, a life. Since the patchwork girl is composed of different people's bodies (as humans are composed of their parents' DNA), her scars are what are most her. Similarly, what constitutes my identity is my unique recombination of my parents' genes, the interweaving of chromosomes that occurs in meiosis.
The scene where Mary exchanges a piece skin from her calf for one of the creature's scars may represent Mary Shelley's miscarriage. The fact that Mary took the skin from her calf is similar to the myth of Dionysus; Zeus carried his unborn son Dionysus in his calf until he was old enough to be born. However, abortion, miscarriage, and birth seem to share similar characteristics in Patchwork Girl, in that they involve an artificial cut between the fetus and the mother. After Mary and the monster have sex, the monster says that Mary had touched her inside herself. Nothing had entered her before except for food, which had already agreed to relinquish its identity and become part of her. Embryos, also exist inside of bodies and hence are those bodies. In a sense the souls of embryos and mothers are shared, since Jackson asserts that the body is the writing of the soul, albeit imperfect, like a charcoal rubbing. Jackson discusses the grisly act of eating embryos, extrapolating the saying "we are what we eat" to her ideas of pregnancy.
The construction of identity is tricky, then, since we do not have it immediately upon conception, our origin. We must take what is given to us -- a name, a template of genes -- and construct something from it. We have a finite number of elements such as letters or genes with which to cut apart and re-stitch. Milan Kundera's Immortality suggests that gestures are a type of this finite pool of elements. Since there are far fewer possible gestures than there are people in the world, gestures wear us instead of the other way around.
On an unrelated note, I found the reference to Tituba's eyeballs puzzling. Tituba was a slave from Barbados who was played an important role in the beginning of the Salem witch trials. She lived in the household of Reverend Parris and looked after his invalid wife. (In Arthur Miller's The Crucible, Goody Parris is dead, not an invalid.) Other possible references to Salem are Nathaniel Hawthorne's shining city on a hill and the graveyard of dead sisters. Different versions of the witch trials state that either Ann Putnam or Betty Parris of Salem conjured up her dead sisters with Tituba's help. If this was who Shelley Jackson was referring to, however, she seems to have deliberately blurred her characters. She writes as if Tituba was the invalid. In a sense, Tituba is the invalid; if Goody Parris taught her to read then her eyeballs belong to Tituba. In any case, Jackson often deludes the reader as to who is the speaking subject -- the creature and Mary are the writing subjects at different times.