Introduction: Carbon monoxide (CO) is a common toxic gas, and CO poisoning in pregnant women can be particularly dangerous, as the fetus is more vulnerable to CO exposure. Even if the mother experiences only mild symptoms and has low carboxyhemoglobin (COHb) levels following exposure, fetal COHb levels can be much higher. Therefore, there is no clear consensus for defining mild CO poisoning during pregnancy. Here, we report a case of preterm delivery following mild CO intoxication. Case report: An 18-year-old female at 36 weeks of gestation visited the hospital with CO poisoning 1 h after exposure. She had only mild headache and COHb level was checked 3.9%. Hyperbaric oxygen (HBO₂) therapy was not provided because she and her husband refused this treatment. Consequently, normobaric oxygen therapy was initiated. Five hours later, her symptoms improved, and thus, the therapy was discontinued. Eight hours later, the patient experienced labor pain, and the baby was delivered by spontaneous vaginal delivery. Neonatal COHb was 1.3% at birth, and the newborn was healthy. Conclusion: If maternal symptoms are mild following low CO exposure, normobaric oxygen therapy or HBO₂ therapy can be considered. However, whatever we choose, we should be prepared for premature birth.