Abstract This paper reviews the scientific evidence for the safety of carbon monoxide (CO) and nitric oxide (NO) inhalation to measure pulmonary diffusing capacity (DLCO and DLNO ) in pregnant women and their fetuses. In eight earlier studies, 650 pregnant women had DLCO measurements performed at various times during pregnancy, with a minimum of two to four tests per session. Both pregnant subjects that were healthy and those with medical complications were tested. No study reported adverse maternal, fetal, or neonatal outcomes from the CO inhalation in association with measuring DLCO . Eleven pregnant women, chiefly with pulmonary hypertension, and 1105 pre-term neonates, mostly with respiratory failure, were administered various dosages of NO (5–80 ppm for 4 weeks continuously in pregnant women, and 1–20 ppm for 15 min to 3 weeks for the neonates). NO treatment was found to be an effective therapy for pregnant women with pulmonary hypertension. In neonates with respiratory failure and pulmonary hypertension, NO therapy improved oxygenation and survival and has been associated with only minor, transient adverse effects. In conclusion, maternal carboxyhemoglobin (HbCO ) levels can safely increase to 5% per testing session when the dose-exposure limit is 0.3% CO inhalation for ≤3 min, and for NO, 80 ppm for ≤3 min. The risk of late fetal or neonatal death from increased HbCO from diffusion testing is considerably less than the risk of death from all causes reported by the Centers for Disease Control, and is therefore considered “minimal risk”.