In a 22‑year‑old G2P1 woman at 30 weeks gestation with obesity (BMI 36), prior gestational hypertension and a family history of preeclampsia, should low‑dose aspirin be initiated for preeclampsia prophylaxis?

Should Low-Dose Aspirin Be Initiated at 30 Weeks?

Yes, initiate low-dose aspirin immediately at 100-150 mg daily and continue until delivery, despite being at 30 weeks gestation—this patient has multiple high-risk factors (prior gestational hypertension, family history of preeclampsia, and BMI 36) that warrant prophylaxis even when started later than the optimal 12-16 week window. 1, 2

Why Aspirin Should Still Be Started Now

This patient meets criteria for aspirin prophylaxis through multiple pathways:

• Prior gestational hypertension is a high-risk factor that independently qualifies her for aspirin prophylaxis 2, 3
• Family history of preeclampsia is a moderate-risk factor 2, 3
• BMI 36 (obesity, BMI >30) is another moderate-risk factor 2, 3
• Having more than one moderate-risk factor (family history + obesity) meets ACOG criteria for aspirin prophylaxis even without the history of gestational hypertension 2, 3

Women with both prior gestational hypertension and family history face significantly elevated risk: the combination of chronic hypertension and history of preeclampsia increases superimposed preeclampsia risk to 45.7% versus 30.1% in those with hypertension alone 4

Optimal Dosing for This Patient

Prescribe 100-150 mg daily (not the standard 81 mg) based on her risk profile:

• BMI 36 reduces aspirin effectiveness due to increased platelet turnover and altered pharmacokinetics; higher doses (100-150 mg) are needed to achieve adequate platelet inhibition in obesity 1, 5
• Prior gestational hypertension may represent underlying chronic hypertension, and standard 81 mg aspirin has shown no benefit in reducing superimposed preeclampsia in women with chronic hypertension (34.3% without aspirin vs 35.5% with 81 mg aspirin, p=0.79) 1
• Evidence demonstrates doses >100 mg are significantly more effective than lower doses when initiated before 16 weeks (RR 0.33,95% CI 0.19-0.57, p<0.0001), and emerging data suggest higher doses may retain benefit even with later initiation 1, 6

Timing Considerations and Expected Benefits

While 12-16 weeks is optimal, starting at 30 weeks still provides meaningful protection:

• ACOG guidelines specify aspirin can be initiated between 12-28 weeks of gestation, with the window extending to 28 weeks precisely because late second-trimester initiation retains benefit 2, 3
• Preeclampsia risk persists and actually increases in the third trimester, making protection during weeks 30-40 clinically valuable 1
• Aspirin reduces preeclampsia risk by 24%, preterm birth by 14%, and intrauterine growth restriction by 20% in high-risk women 7, 4

The pathophysiologic rationale supports late initiation: while defective placentation occurs in the first trimester, aspirin's anti-inflammatory and endothelial protective effects continue to provide benefit by reducing the maternal inflammatory response and improving endothelial function throughout pregnancy 7, 8

Safety Profile and Duration

Continue aspirin daily until delivery without stopping at 36 weeks:

• Low-dose aspirin (75-162 mg) does not increase risks of placental abruption, postpartum hemorrhage, fetal intracranial bleeding, perinatal mortality, or congenital anomalies 7, 1
• Stopping at 36 weeks "just to be safe" is not evidence-based and removes protection during a high-risk period when preeclampsia and eclamptic seizures can develop for the first time, even postpartum 1
• The FDA warning about aspirin in third trimester refers to high doses (>100 mg for analgesic purposes), not prophylactic low doses of 75-150 mg 1

Common Pitfalls to Avoid

Do not dismiss aspirin prophylaxis simply because she is already at 30 weeks:

• The "12-16 week optimal window" does not mean aspirin is contraindicated or ineffective after 16 weeks 2, 3
• ACOG explicitly extends the initiation window to 28 weeks, and at 30 weeks she is only 2 weeks beyond this 2, 3

Do not prescribe standard 81 mg dosing for this patient:

• Her obesity (BMI 36) and prior gestational hypertension specifically require higher dosing (100-150 mg) for effectiveness 1, 5
• Standard 81 mg has failed to show benefit in women with chronic hypertension, and her prior gestational hypertension may represent undiagnosed chronic hypertension 1

Do not plan to stop aspirin at 36 weeks:

• Multiple international guidelines (ACOG, Canadian, Irish, New Zealand, UK) specify continuation until delivery 1
• Preeclampsia risk continues through delivery and into the early postpartum period 1