Malebranche reasons that, since the infant in the womb is part of its mother's body, in complete dependence on its mother, it makes sense to think that the infant has the same passions and sensations as the mother. What the mother experiences, the infant also experiences. This line of thought, Malebranche thinks, is "incontestable for many reasons":
Consider only that a mother who is very frightened at the sight of a cat begets a child with a horror that surprises him every time this animal is presented to him. It is easy to conclude from this that the child must have seen with the same horror and emotions of spirit what its mother saw when she carried it in her womb, since the sight of a cat that does it no harm still produces in it such strange effects. (LO 113)
However, Malebranche is still a Cartesian, and that means he's very cautious about things that are strictly certain. He insists that he is proposing this communication between the mother and the child only as a hypothesis, which he thinks is heavily confirmed by a number of things, and without which he thinks many things remain obscure.
Another element in the discussion is the fact of sympathy or compassion. At one point Malebranche discusses how the look of an impassioned man "penetrates" even people who hardly know him (LO 113), and at another a young maidservant who, on seeing someone bled through the foot, took to bed with a pain in her foot for several days (LO 114). Unborn children, however, have more delicate brain fibers, and thus are more subject to these sorts of sympathy experiences.
On this basis Malebranche explains the case of a young man who was born mad, with his bones broken in the same places that a criminal's bones were broken:
According to the principles just established, the cause of this disastrous accident was that his mother, having known that a criminal was to be broken, went to see the execution. All the blows give to this miserable creature forcefully struck the imagination of this mother and, bya sort of counterblow, the tender and delicate brain of her child....At the sight of this execution, so capable of frightening a woman, the violent flow of the mother's animal spirits passed very forcefully from her brain to all the parts of her body corresponding to those of the criminal, and the same thing happened in the child. (LO 115)
The mother's body is able to withstand these violent tempests of animal spirits, but the child's body was not. Malebranche goes on to connect this case with other cases, like birthmarks:
The explanations of this accident are broad enough to explain how pregnant women who see people marked on certain parts of the face imprint these same marks on their unborn children, and on the same parts of their bodies; and one can judge from this that it is with reason that pregnant women are urged to rub some hidden part of their body when they see something that surprises them , or when they are excited by some violent passion, for that can make the marks appear on these hidden parts rather than on face of their child. (LO 116)
Monstrous births are covered in this explanation, too. The particular example Malebranche gives is interesting:
It has not been more than a year since a woman, having attended too carefully to the portrait of Saint Pius on the feast of his canonization, gave birth to a child who looked exactly like the representation of the saint. He had the face of an old man, as ar as is possible for a beardless child; his arms were crossed upon his chest, with his eyes turned toward the heavens; and he had very little forehead, becasue the image of the saint beign raised toward the vault of the church, gazing toward heaven, had almost no forehead. He had a kind of inverted miter on his shoulders, with many round marks in the places where miters are covered with gems. In short, this child strongly resembled the tableau after which it mother had formed it by the power of her imagination. This is something that all Paris has been able to see as well as me, because the body was preserved for a considerable time in alcohol. (LO 116)
Even stranger things than the Saint Pius baby were birthed; Malebranche goes so far as to say that "there is nothing so bizarre that it has not been aborted at some time," and gives as examples babies shaped like "apples, pears, grapes, and other similar things" (LO 117).
Malebranche thinks that this communication between mother and infant serves some biological functions. For instance, he attributes instincts to it (giving the age-old example of the lamb and the wolf, which is found in medieval authors). Further, he suggests that "without this communication women and animals could not easily bring forth young of the same species" (LO 117). This account of epigenesis is an interesting one. The debate between epigeneticists and preformationists was a major one at the time. Malebranche's account of heredity is actually a blend of preformationist and epigenetic views; in the Dialogues on Metaphysics he talks about all the bees that will ever exist pre-existing in the first bee. This is a preformationist view; but that Malebranche is not purely preformationist is clear from his comment here. He is primarily preformationist, but he gives some credit to the epigenetic side as well. As he goes on to say:
It is true that the most reasonable thinking, that which conforms most closely to experience in this very difficult question of the formation of the fetus, is that infants are already almost completely formed even before the action by which they are conceived, and during the gestation period their mothers do nothing but provide them their normal growth. However, this communication of the mother's animal spirits and brain with those of the infant seems to serve to regulate this growth, determining the particles used to nourish it to be arranged gradually in the same way as in the mother's body; which is to say, this communication of the spirits renders the child like its mother, or of the same species. (LO 117-118)
Malebranche thinks this is confirmed by mutations like the fruit-shaped miscarriages; although he allows that God might have set things up in a purely preformationist way. It's not impossible that the purely preformationist account is the right one, but the evidence - monstrous births and tulips (plants from the bulb are more similar to the mother plant than plants from the seed) - suggests a certain amount of epigenetic regulation.
The communication between mother and child plays a role in Malebranche's moral psychology, since he attributes a number of errors to it. One set of errors are what might be called temperamental errors, i.e., excessive domination by a given passion. Malebranche gives as an example the case of King James, which I confess to finding rather funny:
One might nevertheless relate here the example of King James of England, of whom Sir Kenelm Digby speaks in his book on the Power of Sympathy. He assures us in this book that when Mary Stuart was pregnant with King James, some Scottish Lords entered her chambers and killed her secretary, who was Italian, in her presence, even though she threw herself in front of him in an effort to obstruct them. This princess received some minor wounds, and her fright so impressed her imagination that they were communicated to the child she carried in her womb. Thus, her son King James remained incapable all his life of looking at a naked sword. Digby says he experienced this himself when he was knighted, for when this prince began to touch his shoulder with the sword, he moved it directly toward his face, and he would even have been wounded had someone not adroitly guided it to where it belonged. (LO 119-120)
It is this sort of thing, Malebranche thinks, that explains phobias.
More important than temperamental errors, however, is the Error of all errors: original sin. Malebranche appeals to the communication between mother and child to explain the transmission of concupiscence, i.e., excessive attachment to sensible things, which is what constitutes original sin. Original sin, I would argue, plays a key role in Malebranche's epistemology, so although Malebranche is not strictly committed to his explanation of its transmission, it does nonetheless have an important role to play.
(Sharon suggests that there could be a sort of series on this, and I agree - at the very least we could get up enough posts for a Sideshow in the next Carnivalesque or History Carnival, all on the theme of pregnancy, birth, and miscarriage in the early modern period. So, if you know of anything that you could blog on the subject, go ahead and do it. The more the merrier!)