The belief that the mother is completely responsible for the sex of the child is wrong because the child

Both father and mother contribute one sex chromosome each to the offspring. Mother has two X chromosomes whereas father has one X and one Y chromosome. So the offspring obtain one X chromosome from mother and whether X or Y chromosome inherited from father will determine the sex of the child. If X chromosome is inherited from father the offspring will be a girl but if it is Y then the baby will be a boy.