In the aetiology of postpartum uterine atony, hypoxia is considered an important factor although some suggest that peripheral oxygen saturation is not influenced by oxygen inhalation in women during the first and second stages of labor. Enhancing oxygen delivery to myometrium through additional inhaled oxygen may improve uterine contractions. Therefore, it is reasonable to consider that oxygen inhalation may promote myometrial contraction and prevent postpartum haemorrhage (PPH) due to uterine atony. The tendency for the uterus to relax in women encountering respiratory problems immediately after cesarean section under general anaesthesia further strengthened this theory.

The aim of this study was to evaluate the effectiveness of oxygen inhalation immediately after vaginal delivery on blood loss. The investigators hypothesized that inhaled oxygen helps to maintain uterine retraction during immediate postpartum period and hence reduces vaginal blood loss.

Inclusion Criteria:

• gestational age between 37 and 42 weeks;
• singleton pregnancy;
• live fetus;
• cephalic presentation;
• neonatal birth weight of 2500-4500 g;
• parity between one and five;
• maternal age < 35 years old;
• vaginal birth

Exclusion Criteria:

• blood pressure ≥ 140/90mmHg;
• placenta previa;
• placental abruption;
• a history of any bleeding during pregnancy;
• a history of curettage;
• cesarean section or any uterine scar;
• a history of postpartum hemorrhage;
• polyhydramnios;
• signs or symptoms of maternal infection;
• known uterine anomalies;
• history of any drug use during labor;
• abnormal placentation;
• coagulation defects;
• instrumental deliveries;
• hemoglobin concentration < 8 g/dL;
• history of anticoagulant drugs;
• beta-mimetic medications during pregnancy;
• prolongation of the first stage of labor > 15 hours