KAILUA KONA, HAWAII — Insignificant trauma to the mother may not be insignificant to the fetus, Dr. William G. Barsan said at a conference on obstetrics, gynecology, perinatal medicine, neonatology, and the law.
Severe maternal injury is likely to lead to fetal loss in 40%–50% of cases, but severe maternal injuries are relatively rare. Minor injuries to the mother result from 88% of trauma in pregnancy; 60%–70% of fetal losses resulting from maternal trauma follow relatively minor maternal injuries, said Dr. Barsan, professor and chair of emergency medicine at the University of Michigan, Ann Arbor.
Placental abruption is the cause of fetal death in 50%–70% of losses after maternal trauma. “This is the one that may occur with relatively minor trauma” and can be hard to detect, he said.
In one study of nine fetal deaths after 25-mph motor vehicle crashes in which the mothers were not wearing seat belts, six of the women sustained only “insignificant” injures, such as bruising or abrasions, Dr. Barsan noted.
A separate study of 22 fetal deaths resulting from motor vehicle crashes found that six mothers sustained no injuries at all, and nine had bruised abdomens. Other maternal injuries included three ruptured uteri, two chest injuries, one extremity fracture, and one head injury with shock.
Perform electronic fetal monitoring for 4 hours on any pregnant woman with a viable fetus who sustains a significant impact to the torso from falling, crashing, or other causes, Dr. Barsan advised. In one study, all patients with placental abruption after trauma developed uterine contractions every 2–5 minutes at some point during a 4-hour monitoring period.
Many women will have uterine contractions after trauma, and most will not have placental abruption. At Dr. Barsan's institution, women with frequent uterine contractions after trauma receive an additional 24 hours of electronic fetal monitoring.
“This seems to be a protocol that works pretty well” to identify patients at risk of placental abruption, he said at the conference sponsored by Boston University.
Even if the patient says that she fell yesterday, or last night, do 4 hours of monitoring, he added. Traumas unrelated to the torso—such as hammering a finger—do not call for monitoring.
If there are no adverse outcomes within the first few days after trauma, pregnancy outcomes can be expected to be similar to cases without trauma, he said.
Besides placental abruption, maternal hypovolemic shock kills less than 5% of fetuses after maternal trauma; direct fetal injury causes less than 10% of deaths, and about 10% of fetuses die because the mother died after trauma.
No cause is recognized in more than 10% of fetal deaths after maternal trauma, he said.
The cause of placental abruption in motor vehicle accidents was demonstrated in crash testing using “pregnant” dummy-within-a-dummy models and computer modeling.
A frontal impact first throws the uterus forward against the abdominal wall, increasing anterior intrauterine pressure up to 550 mm/Hg. Then the torso gets thrown forward and the body flexes forward, crushing the uterus between the torso and the knees and causing a second increase in intrauterine pressure, which may become as great as 600 mm/Hg.
All this creates a high degree of negative pressure in the back of the uterus that can pull the placenta off the uterine wall, Dr. Barsan explained.
'[Placental abruption] is the one that may occur with relatively minor trauma' and can be hard to detect. DR. BARSAN
Fetal Loss From Maternal Trauma
U.S. deliveries per year 4 million
Pregnancies complicated by trauma 6%–7%
Fetal loss in pregnancies with trauma 1%–2%
Number of fetuses lost from trauma 2,600–5,200
Source: Dr. Barsan