Why is antihypertensive therapy not initiated in the third trimester for chronic hypertension (CHTN) in pregnancy with elevated blood pressure?

Antihypertensive Therapy in Third Trimester Chronic Hypertension: Rationale for Selective Treatment

Antihypertensive therapy is not routinely initiated for mild-to-moderate chronic hypertension in the third trimester because treatment has not been shown to prevent preeclampsia, improve perinatal mortality, or enhance fetal outcomes despite reducing the risk of progression to severe hypertension. 1

Understanding the Evidence Base

The management of chronic hypertension during pregnancy requires careful consideration of both maternal and fetal risks. The current evidence provides several key insights:

• Antihypertensive treatment for mild-to-moderate hypertension (140-160/90-110 mmHg) reduces the risk of progression to severe hypertension by approximately 50% compared to placebo 1
• However, treatment has not been shown to:
  • Prevent preeclampsia
  • Reduce preterm birth rates
  • Prevent small-for-gestational-age infants
  • Improve infant mortality 1

Treatment Thresholds and Recommendations

When to Treat Chronic Hypertension in Pregnancy

• Severe hypertension (≥160/110 mmHg): Treatment should be initiated immediately to prevent maternal cerebrovascular complications 1, 2
• Mild-to-moderate hypertension (140-159/90-109 mmHg): The evidence for treatment is less clear 1

Treatment Algorithm

1. Assess severity of hypertension:

   • BP <140/90 mmHg: No treatment needed
   • BP 140-159/90-109 mmHg: Consider individual risk factors
   • BP ≥160/110 mmHg: Immediate treatment required

2. For mild-to-moderate hypertension, consider:

   • Presence of end-organ damage
   • Risk of preeclampsia
   • Gestational age
   • Maternal comorbidities

3. If treatment is indicated, preferred medications:

   • Methyldopa (first-line due to extensive safety data) 1
   • Labetalol (good safety profile, especially in third trimester) 1
   • Nifedipine (calcium channel blocker with established safety) 1

Recent Evidence Challenging Traditional Approach

The 2022 CHAP trial 3 has provided new evidence that treating mild chronic hypertension (targeting BP <140/90 mmHg) may be beneficial:

• 18% reduction in adverse pregnancy outcomes (preeclampsia with severe features, medically indicated preterm birth <35 weeks, placental abruption, or fetal/neonatal death)
• No increase in small-for-gestational-age births
• 21% reduction in preeclampsia
• 13% reduction in preterm birth

Potential Risks of Treatment

• Excessive BP reduction may impair uteroplacental perfusion 1
• Some medications have specific risks:
  • ACE inhibitors and ARBs are contraindicated throughout pregnancy (fetotoxicity) 1
  • Beta-blockers (particularly atenolol) may cause intrauterine growth restriction when used for extended periods 4
  • Diuretics may reduce plasma volume expansion 1

Clinical Pitfalls to Avoid

1. Don't use ACE inhibitors or ARBs at any point during pregnancy due to fetotoxicity 1

2. Don't aggressively lower BP below 130/90 mmHg as this may compromise uteroplacental perfusion 2

3. Don't miss superimposed preeclampsia which develops in 20-25% of women with chronic hypertension and significantly increases maternal and fetal risks 1

4. Don't forget to monitor fetal growth as some antihypertensive medications may affect fetal development 4

5. Don't delay treatment of severe hypertension (≥160/110 mmHg) as this increases risk of maternal cerebrovascular complications 2

Conclusion

While the traditional approach has been to reserve antihypertensive treatment in pregnancy for severe hypertension, emerging evidence suggests potential benefits of treating mild-to-moderate chronic hypertension. However, the decision to initiate therapy in the third trimester should carefully weigh the modest benefits against potential risks to fetal development and uteroplacental perfusion.