Retained Placental Fragments: See "Tissue" section below for detailed mechanism. When parts of the placenta remain, the uterus cannot contract fully.

Uterine Fibroids: Benign (non-cancerous) growths in the uterine wall.

Mechanism: Fibroids can interfere with the normal arrangement and function of the myometrial muscle fibers, preventing uniform and effective contractions.

Magnesium Sulfate Administration: Used to prevent seizures in pre-eclampsia or for fetal neuroprotection.

Mechanism: Magnesium is a smooth muscle relaxant. It reduces calcium influx into muscle cells, which is essential for muscle contraction. This effect extends to the uterine musculature, decreasing its ability to contract.

General Anesthesia (especially halogenated agents like halothane, isoflurane):

Mechanism: These agents are potent smooth muscle relaxants, directly inhibiting uterine contractility.

Placenta Previa or Abruptio Placentae: Conditions where the placenta is abnormally implanted or separates prematurely.

Mechanism: These conditions can lead to a larger than normal placental implantation site or damage to the uterine wall, making it harder for the uterus to contract effectively post-delivery.

2.2. Trauma (Genital Tract Trauma)

This refers to lacerations (tears) or hematomas (collections of blood outside blood vessels) in the birth canal (cervix, vagina, perineum) or uterus itself. This is the second most common cause of PPH.

Mechanism:
- Direct injury to blood vessels during delivery results in active bleeding.
- Unlike uterine atony where bleeding is from the placental site, here the bleeding source is the site of the tear.
- The uterus may be well-contracted, but blood continues to flow from the laceration.
Types and Risk Factors:
- Cervical Lacerations: Tears in the cervix.
  - Risk Factors: Forceps or vacuum-assisted delivery, rapid cervical dilation, large baby.
  - Mechanism: The cervix is highly vascular. A tear can involve significant blood vessels, leading to profuse bleeding.
- Vaginal or Perineal Lacerations: Tears in the vagina or perineum (the area between the vagina and anus).
  - Risk Factors: Episiotomy (surgical cut to enlarge the vaginal opening), instrumental delivery, rapid delivery, large baby.
  - Mechanism: Tears can extend deeply into highly vascular tissues, causing significant blood loss.
- Uterine Rupture: A tear in the wall of the uterus, a rare but life-threatening event.
  - Risk Factors: Previous C-section (scar rupture is most common), prolonged labor, excessive oxytocin stimulation, high parity, uterine anomalies.
  - Mechanism: The integrity of the uterine wall is compromised, leading to massive internal and/or external hemorrhage. Blood can escape into the abdominal cavity.
- Hematomas: Collections of blood, usually in the vulva, vagina, or retroperitoneum (space behind the abdominal lining).
  - Mechanism: Blood vessels are damaged, and blood leaks into surrounding tissues, forming a localized swelling. This can mask significant blood loss, as the