ABSTRACT
Anatomy Early in the gestational period, the fetus is well protected in the uterus which lies within the pelvis. After the 12th week, the uterus begins to expand out of the pelvis and into the abdomen reaching the umbilicus at 20 weeks gestational age and the costal margin at 36 weeks.(12, 15) In the final 4 weeks, the fetus descends into the pelvis and fundal height is lower. As pregnancy advances, maternal organs are displaced upward and the diaphragm may be elevated as much as 4 cm from the nonpregnant state. While the gravid uterus may provide a degree of protection to the maternal viscera, the fetus is at increased risk of direct injury as the pregnancy progresses. In the third trimester, the uterine wall thins, the amount of amniotic fluid decreases, and the fetal head rotates down and into the pelvis all of which place the fetus at increased risk for injury.(12, 15-16) In particular, the location of the fetal head within the pelvis places the fetus at risk of skull fractures and traumatic brain injury secondary to maternal pelvic fractures.
