The investigators have design a randomized double blind clinical trial to know the incidence of breaktrough pain (BP) in high-risk of BP parturients (nulliparous with early cervical dilation) comparing two epidural analgesic regimes: programed intermittent epidural boluses versus patient controlled epidural analgesia. The role of the epidural technique (epidural versus combined spinal-epidural) in the incidence of BP will be also evaluated.

Women in labor receiving epidural analgesia may experience breakthrough pain (BP) that requires the use of additional medication and that is a source of dissatisfaction with the analgesic treatment for labor.

The investigators can define two types of BP: 'primary breakthrough pain' is the first moment when the patient requests analgesia during labor. 'Secondary breakthrough pain' can be defined as the time a previously effective epidural analgesia turns ineffective.

The BP may have multiple causes, such as delivery progression and catheter misplacement. There is no single pattern to provide adequate neuraxial analgesia for labor, but for certain patients and specific clinical situations some methods provide advantages over others.

The choice of a suitable system of epidural drug delivery has an important role in getting proper analgesia and a low incidence of breakthrough pain during labor. In order to develop preventive strategies for BP, some clinical scales to estimate its incidence rate have been described. Among risk factors for BP the investigators can include: Nulliparity, epidural catheter placement at an earlier cervical dilation, neonatal weight and technique performed (epidural versus combined spinal-epidural - CSE).

In addition, technological development has offered us new treatment modalities of pain management during childbirth have evolved recently to improve maternal satisfaction, as well as to treat and prevent BP: Patient Controlled Epidural Analgesia (PCEA) added or not to a continuous epidural infusion (CEI), computer integrated PCEA (CI-PCEA) and automatic or programed intermittent epidural boluses (PIEB).

Nowadays, there is no evidence about the best epidural analgesic treatment or the optimum epidural punction in order to reduce BP episodes and, hence, increase maternal satisfaction.

Therefore, the investigators have design a randomized double blind clinical trial to know the incidence of breaktrough pain (BP) in high-risk of BP parturients (nulliparous with early cervical dilation) comparing two epidural analgesic regimes added to a contiuous infusion of L-bupivacaine plus fentanyl: programed intermittent epidural boluses versus patient controlled epidural analgesia. The role of the epidural technique (epidural versus combined spinal-epidural) in the incidence of BP will be also evaluated.

Inclusion Criteria:

1. Age between 18 and 40 years
2. Gestational age between 37 and 41 weeks
3. Singleton pregnancies
4. Nulliparity
5. Spontaneous or induced labor
6. Cervical dilation less than four centimeters

Exclusion Criteria:

1. Do not meet all the inclusion criteria
2. Do not consent to the study
3. Systemic medical pathologies as pre-eclampsia, insulin-treated gestational diabetes or multiple sclerosis that could influence medical decision during labour and/or make randomization unfeasible
4. Contraindication for neuraxial analgesia