sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: The fetus' brain begins to develop at the beginning of the 24th week of pregnancy, which may have a link to why the cysts resolve themselves by the 26th week.
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?