sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: At about  six weeks in the male fetus, the Y chromosome causes the testes to develop and to begin to produce the male hormone testosterone.
sentence2: The development of the respiratory system in the fetus begins at about 4 weeks.
question: The development of the respiratory system in the fetus begins at about?