“A Hatred So Intense: We Need to Talk About Kevin, Postfeminism and Women’s Cinema” by Sue Thornham over at Sequence (an experimental, peer-reviewed, media, film and music studies serial publication) is really worth the read. It’s very heavy with academic language, but none the less interesting.
The idea of the child as monstrous double of the mother is one that has been explored in two very different places: in feminist writing and in the horror film. Adrienne Rich writes of ‘the dread of giving birth to monsters’ (1977: 164) and Phyllis Chesler calls her unborn child ‘my monster, myself’, wondering ‘What if you’re born … with my anger, my excesses?’ (1998: 36, 101). For Rich, such anxieties are the product of patriarchal associations of childbirth with evil and the resulting internalised feelings of guilt – she points to the prevalence across cultures of notions of the female body as ‘unclean, and as the embodiment of guilt’ (1977: 164). She also points to women’s repressed anger at the death of self which accompanies motherhood, quoting the following diary extract from Elizabeth Mann Borgese’s Ascent of Woman:
My face in the mirror looked alien to me. My character blurred. Childish violent desires, unknown to me, came over me, and childish violent dislikes. I am a coldly logical thinker, but … my reasoning blurred and dissolved… I was one and the other at once. It stirred inside of me. Could I control its movements with my will? Sometimes I thought I could, at other times I realized it was beyond my control. I couldn’t control anything. I was not myself. And not for a brief passing moment of rapture, which men, too, may experience … Then it was born. I heard it scream with a voice that was no longer mine. (Borgese 1963: 45)
Lucy Fischer draws upon such accounts in her analysis of Polanski’s Rosemary’s Baby (1968). She argues that the film acts as a ‘skewed “documentary”’ for its age, recording not only patriarchal horror at the maternal body and the birth process but also, and against the grain of much feminist writing of the time, ‘women’s private experience of pregnancy’ (ibid.: 415). As applied to Polanski’s film this seems to me to be a questionable argument, yet it is clear that the sense of maternal splitting and alienation that in the horror film generates the monstrous child has also been a key but repressed part of women’s experience of maternity.
(Thanks to @tjinimin for the link).

No mention in the article of Frankenstein, the ultimate monstrous birth anxiety myth.