An embryo develops over several weeks in the womb, going from a single cell to a fully formed human. In the early stages it receives nutrients from the yolk sac, then the umbilical cord. By 8 weeks limbs have begun to form and by 16 weeks the baby can move its hands and feet, though the mother cannot feel it. Advanced 4D imaging allows viewing facial expressions by 24 weeks when the lungs are nearly complete and survival outside the womb is possible. By 40 weeks of gestation the baby is ready to be born, completing the miracle of forming a human life in the womb.