At what point in the gestation cycle is a pulmonary embolism (PE) most likely to occur in pregnant women?

Timing of Pulmonary Embolism During Pregnancy

The risk of pulmonary embolism (PE) is present throughout pregnancy but peaks during the postpartum period, particularly within the first 6 weeks after delivery. 1

Risk Distribution Throughout Pregnancy

• The risk of venous thromboembolism (VTE) is spread across all three trimesters of pregnancy, with the risk appearing highest in the third trimester 1
• While the absolute incidence of VTE is equal during antepartum and postpartum periods (approximately 0.6 per 1000 pregnant women for each period), the daily risk is significantly higher postpartum due to the shorter timeframe 1
• The postpartum period, especially the first 6 weeks after delivery, represents the time of greatest risk for PE development 1, 2
• About half of severe postpartum PEs occur within the first 24 hours after delivery 3

Epidemiology and Risk Factors

• PE remains one of the leading causes of maternal death in high-income countries, with mortality rates of approximately 1.13 per 100,000 maternities 1
• The incidence of PE during pregnancy is approximately 1 in 1000 pregnancies 4
• The mortality rate for pregnant women with PE is about 3%, which is significantly higher than for non-pregnant women with PE 4
• By the end of 4 weeks postpartum, the weekly rate of PE approaches the background rate in the general population 2

Risk Factors That Increase PE Likelihood

• Prior history of VTE significantly increases risk during pregnancy 1
• In vitro fertilization increases VTE risk with an HR of 1.77 overall and 4.22 during the first trimester 1
• Cesarean section delivery increases risk compared to vaginal delivery, though the absolute event rate remains low 2
• Other significant risk factors include:
  • Stillbirth (adjusted OR = 5.97) 2
  • Lupus (adjusted OR = 8.83) 2
  • Transfusion of coagulation products (adjusted OR = 8.84) 2
  • Obesity 1
  • Medical comorbidities 1
  • Pre-eclampsia 1
  • Post-partum hemorrhage 1

Physiological Basis for Increased Risk

• Pregnancy is a hypercoagulable state due to hormonal, mechanical, and blood composition changes 1
• Mechanical compression of iliac veins by the enlarged uterus reduces femoral venous blood flow, with a more marked effect on the left side (explaining higher incidence of left-sided DVT) 1
• Coagulation factors II, VII, and X increase by the third trimester 1
• Levels of coagulation inhibitor proteins decrease during pregnancy 1
• Plasma fibrinolytic activity decreases during pregnancy 1
• Hypercoagulability persists until approximately 2 weeks after delivery 1

Clinical Implications

• PE diagnosis during pregnancy can be challenging as symptoms frequently overlap with normal pregnancy symptoms 1
• The prevalence of confirmed PE is low (2-7%) among pregnant women investigated for the disease 1
• D-dimer levels continuously increase during pregnancy, with levels above the VTE "rule-out" threshold in almost 25% of pregnant women in the third trimester 1
• Healthcare providers should maintain a high index of suspicion for PE throughout pregnancy, but particularly in the immediate postpartum period 1, 2

Understanding the timing of PE risk during pregnancy is crucial for appropriate vigilance, early diagnosis, and timely intervention to reduce maternal morbidity and mortality.