When any woman by any wilful act or omission causes the death of her child being a child under the age of 12 months, but at the time of the act or omission the balance of her mind was disturbed by reason of her not having fully recovered from the effect of giving birth to the child or by reason of the effect of lactation consequent upon the birth of the child, she shall, notwithstanding that the circumstances were such that but for this section the offence would have amounted to murder, be guilty of the offence of infanticide.