When does surfactant production stas in the lungs of a fetus?

Surfactant production stas between 20 and 24 weeks. It appears in amniotic fluid between 28 and 32 week. Mature levels of pulmonary surfactant are usually present after 35 weeks. At 20 weeks of gestation, fetus has the capability to survive outside the uterus. Ref: Review of Medical Physiology, 22nd Edition By Wiiliam F Ganong, Page 656; Foundations of Maternal and Pediatric Nursing By Lois White, 3rd Edition, Page 9; Pediatric Thoracic Surgery By Dakshesh Parikh, Page 24.