While the initial assessment of the pregnant patient who has sustained trauma should also take into account the fetus, the patient’s welfare and stability are paramount. The rate of fetal mortality after maternal blunt trauma ranges from 3.4% to 38.0%. Most commonly, this results from placental abruption, maternal shock or maternal death. Fetal loss can occur even when the patient has not sustained any abdominal injuries. For this reason, regardless of the apparent severity of injury in blunt trauma, all pregnant women must be evaluated. Given the fact that blunt trauma is relatively common in pregnancy, the pregnant patient should be reminded during prenatal care that if she sustains any form of trauma during pregnancy, she should report this immediately to her physician.

The most important cause of injury to the fetus is abruptio placentae. In one large series of trauma cases, two-thirds of fetal deaths were due to abruption. Abruption complicates 1–5% of minor trauma cases and 20–50% of major accidents. Although abruption may not manifest clinically for several days after the traumatic event, there will almost always be subtle warning signs within hours of the event. These include abdominal and/or uterine tenderness, vaginal bleeding or increased uterine activity. It is vitally important to realize that the diagnosis of placental abruption is a clinical diagnosis based on patient symptoms and abnormalities within the fetal heart rate pattern and/or uterine contraction pattern. Ultrasound misses approximately 50–80% of placental abruption. Therefore, an ultrasound should never be ordered in order to “rule out placental abruption.” When placental abruption is severe enough to result in fetal death, clinically evident coagulopathy will be present in 40% of cases. Other placental injuries that have been described after blunt trauma include fracture of the placenta and disruption of fetal vessels on the surface of the placenta.

Direct injury to the uterus is another important risk of abdominal trauma unique to pregnancy. This may manifest as:

• contusion of the uterus with tenderness on examination
  
• serosal hemorrhage or abrasion
  
• avulsion of the uterine vasculature resulting in intraperitoneal or retroperitoneal hemorrhage
  
• uterine rupture with or without extrusion of the fetus or placenta into the abdominal cavity.

The presentation of uterine rupture may vary from mild uterine tenderness and an abnormal FHR tracing with normal vital signs to rapid onset of maternal hypovolemic shock with fetal and maternal death. Direct injury to the fetus is an uncommon but serious complication of blunt abdominal trauma. Fractures of the fetal skull and extremities have been reported. In addition, direct abdominal trauma may result in fetal subdural, sabgaleal, and intracranial hemorrhages.

Fetomaternal hemorrhage (FMH) may occur independently or in association with the above described injuries. In the majority of cases, the FMH is small and without clinical significance. However, large hemorrhages, which result in a severely anemic newborn or fetal death, may occur. The degree of FMH is difficult to predict. Neither the severity of the trauma nor the mechanism of injury correlates well with the incidence or severity of the hemorrhage. The amount of FMH can be roughly calculated by performing a Kleihauer–Betke (KB) test which is used to identify the number of fetal cells that are present in the maternal circulation. Realizing that the average fetal blood volume is approximately 80 cc/kg, it becomes clear that it does not take much of a FMH before the fetus becomes hemodynamically compromised. In this instance there is usually evidence of abnormalities noted on FHR monitoring such as late FHR decelerations or fetal tachycardia. It is recommended that the KB test be performed in all cases of life-threatening abdominal trauma, whenever there is an abnormal FHR tracing after trauma or when the patient is Rh negative. When neither of these factors is present, the likelihood of finding clinically significant trauma is so low that the KB test is not helpful.

Penetrating abdominal trauma occurs less frequently in pregnancy than blunt trauma. The majority of penetrating trauma is due to gunshots and stab wounds. After mid pregnancy, the uterus is injured more than twice as often as other maternal internal organs. The myometrium, amniotic fluid, fetus, and placenta act as a cushion to protect the other abdominal viscera. However, when the uterus is injured by penetrating wounds, the fetus is injured approximately two-thirds of the time. The damage caused by a bullet is unpredictable and involves many factors. Bullets have a tendency to cause much more intra-abdominal damage that suggested by the small entry wound. Therefore, most authorities recommend exploratory laparotomy for the pregnant patient who sustains a gunshot wound to the abdomen. Stab wounds, on the other hand, do not always require surgical exploration.