Pulmonary embolism is one of the leading causes of maternal mortality.
There is disagreement among different medical societies about the value of D-dimer as a screening modality. If you use it, consider the rational D-dimer approach whereby you add 250 to your cut-off for every trimester.
A useful screening modality is an ultrasound of bilateral lower extremities looking for DVT.
Keep in mind, both a V/Q scan and CT scan have a significant amount of radiation. CTA is probably the right diagnostic test (less radiation than CT w&w/o).
Always use the shared decision-making model and clinical acumen to choose your tests.
Leung AN, et. al. (2011). An official American Thoracic Society/Society of Thoracic Radiology clinical practice guideline: evaluation of suspected pulmonary embolism in pregnancy. American Journal of Respiratory and Critical Care Medicine. 184(10):1200-8
Polak JF, Wilkinson DL. (1991). Ultrasonographic diagnosis of symptomatic deep venous thrombosis in pregnancy. American Journal of Obstetrics and Gynecology. 165(3):625-9.
Sachs BP, et. al. (1987). Maternal mortality in Massachusetts. Trends and prevention. New England Journal of Medicine. 316(11):667-72.