You're right, of course. It's perfectly possible even now for women, particularly teenagers like Esme, to be unaware that they are pregnant.
And, yes, right again. She's not insane and has no mental illness whatsoever. She's saner than most of the other people in her family. She's misdiagnosed as a schizophrenic, which a great number of women were in the last century. What we'd now recognise as normal, teenage, rebellious behaviour was often seen as evidence of insanity, particularly in women. If anyone's interested in this subject, I'd recommend the two books I mention inthe acknowlegements at the back of the novel - the RD Laing and the Elaine Showalter. They explain it better and in rather more detail than me.
Maggie