sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: Despite antibiotic and antifungal treatment, respiratory disease and anorexia developed and the dolphin died 4 weeks after the onset of illness.
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?