Aortocaval compression occurs often when gestational age is > 20 weeks. This compression significantly compromises the chances of maternal survival in cardiac arrest. Because it is often difficult to know the exact gestational age, it is commonly recommended that emergency C-section in maternal cardiac arrest be performed when the fundus extends above the level of the umbilicus.
References
Vanden Hoek TL, et al. Part 12: Cardiac arrest in special situations: 2010 American Heart Association guidelines for cardiopulumonary resuscitation and emergency cardiovascular care. Circulation 2010;122[suppl 3]:S829-S861.
