Cesarean section commonly induces moderate to severe pain for 48 hours. These patients have additional compelling reasons to provide adequate pain relief as early mobilization is a key factor to prevent the risk of thromboembolic event which is increased during pregnancy. Beside these, patients need to be pain free to takecare for their newborn and breastfeed them. Poorly controlled pain after cesarean section also increases risk of chronic pain and postpartum depression.

Intrathecal morphine is considered the "gold standard" for postoperative pain relief after cesarean delivery. The duration of analgesic effect of morphine extend to 12-24 hours. Its widespread use is due to its favorable pharmacokinetic profile, ease of administration and low cost. Although intrathecal morphine is highly effective, its use is associated with undesirable adverse effect particularly nausea, vomiting and pruritus which reduce overall patients' satisfaction. More serious complication is the risk of delayed maternal respiratory depression.

The Quadratus Lumborum block was first described in 2007 which demonstrates a spread to the paravertebral space, thus leads to a more extensive block to T5-L1 nerve branches and a long lasting block with the potential to provide visceral pain relief. Therefore, this block has an evolving role in postoperative analgesia for many lower abdominal surgeries. As the safety is concerned, there has been one report of a patient with unilateral hip flexion and knee extension weakness leading to unplanned overnight admission following lateral quadratus lumborum block after laparoscopic gynaecological operation.

If the result favors effective, it will have the advantage of a combination with intrathecal opioid to prolong the pain free period after cesarean section which has about 4,000 cases per year.

Inclusion Criteria:

• All singleton pregnant women with gestation of at least 37 weeks scheduled for elective cesarean section with American Society of Anesthesiologists (ASA) physical status 1 or 2

Exclusion Criteria:

1. refuse to receive spinal block
2. allergy to drugs used in research: morphine, local anesthetic drug and paracetamol
3. abnormal coagulopathy: congenital coagulopathy or who used anticoagulants
4. platelet dysfunction or thrombocytopenia
5. distorted anatomical structures of lumbar spines
6. systemic infection or local infection at both flank areas which are the punctures sites for quadratus lumborum block
7. unable to comprehend or use the verbal rating pain scoring system or patient-controlled analgesia