At which week do fetal lungs start to mature and by which week they are completed?

Long answer: Fetal lungs go through 5 major stages beginning with the embryonic stage when the primordial lung buds form at 4 weeks of gestation, and end with the alveolar phase at about 36 weeks as the luns start to form the many millions of tiny air sacs known as alveoli. Lung maturation continues after birth until almost 8 years of age when adult type of lungs are fully formed. The whole answer is in books