Epidural space localization using loss of resistance technique with air (ALOR) is controversial in obstetric analgesia due to a minor efficacy and risk of complications, compared with loss of resistance technique with saline (SLOR). This randomized prospective study will compare the efficacy and incidence of most common complications of both techniques in obstetric analgesia.

A local ethics committee has approved the study. A statistical power calculation was performed and 177 patients per group will be necessary to confirm a 20% difference of efficacy of the block at 30 minutes (α=0,05; β=0,1). Epidural catheter will be inserted and a standardized analgesic protocol will initiate. The efficacy of the block will be assessed after 30 minutes and at delivery. The apparition of adverse effects during puncture and labour will be annotated. The need for repuncture of the block and the repercussion of the technique on the delivery and the fetus will be recorded as well. At 24 hours, investigators will assess the incidence of pain at the site of puncture (PSP) and the level of maternal satisfaction on a 0 to10 numeric scale.

Inclusion criteria:

• Parturients asking for epidural analgesia during labour .
• Age between 18 and 45 years.
• Spontaneous or induced.
• Cervical dilatation inferior to 6 cm.
• Written informed consent.

Exclusion criteria:

• Infection in the puncture site.
• Refusal of the patient.
• Coagulopathy.
• Severe hypovolaemia.
• Intracranial hypertension.
• Severe aortic or mitral stenosis.
• Systemic infection.
• Non collaborating patient.
• Incapacity of communicating or lack of comprehension of the study by the parturient
• Prior neurological symptoms or demyelinising lesions.
• Valvular stenosis.
• Severe spinal deviation.
• Patient previously operated of spinal surgery in the site of puncture.