Im what trimester does the respiratory system mature

The second trimester is when the respiratory system is mature. 

The respiratory system matures in the third trimester.

In the 8th week the lungs will start to form.

In the 26th week Air sacs form in baby's lungs, but lungs are still not ready to work outside the womb.

In weeks 27-30 The respiratory system, while immature, produces surfactant. This substance helps the air sacs fill with air.

in weeks 31-34 Rhythmic breathing occurs, but baby's lungs are not fully mature.

In the third trimester, Some fetal systems, such as the respiratory system, only now begin to mature just prior to birth.

Physicians can test the infants lung maturity prior to birth if the mother going into labor.