The Buddha Shakya is conceived with the mind as his lodging

"In turbid water the moon does not abide; in a withered tree there are no birds. In the body of a woman without a mind [of faith] the Buddha does not abide. A woman who keeps the Hokekyo is like clear water: the moon of the Buddha Shakya takes up residence [there]. For example, although, when a woman is first pregnant, she is not aware of it in her own body, as the months gradually add up and the days pass, she first wonders if it is so and later thinks that it is certain. A woman of insight even knows whether it is a boy or a girl. The doctrine of the Hokekyo is also thus. When one believes Namu Myoho renge kyo in one's mind, the Buddha Shakya is conceived with the mind as His lodging. Though at first one does not know it, as the months gradually add up, the Buddha in one's mind appears and a joyful mind gradually emerges." -- Letter to Lord Matsuno's Wife