sentence2_structure: The development of the respiratory system in the fetus<>begins<>at about 4 weeks
sentence1: Completion of this stage, at eight weeks, ensures the body systems are developed and from then on, the embryo is called the fetus.
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?