Canadian Guidelines for ASA in Pregnancy
Primary Indication: Prevention of Pre-eclampsia
Low-dose acetylsalicylic acid (ASA) is recommended during pregnancy to reduce the risk of pre-eclampsia in women at high risk, with treatment initiated before 16 weeks gestation at a dose of 75-162 mg daily. 1, 2
Risk Stratification and Patient Selection
Initiate ASA in pregnant women with lupus nephritis to reduce the risk of pre-eclampsia, as this represents a high-risk condition requiring prophylaxis 1
Consider ASA for women with antiphospholipid syndrome in combination with low molecular weight heparin to prevent pregnancy loss and thrombotic complications 3
Offer ASA to women with abnormal uterine artery Doppler findings identified in early pregnancy, as this indicates abnormal placentation and increased pre-eclampsia risk 4
Prescribe ASA for women with chronic hypertension during pregnancy, as they have elevated risk of superimposed pre-eclampsia and adverse fetal outcomes 2, 5
Timing and Dosing Protocol
Start ASA before 16 weeks gestation to achieve maximum benefit, as initiation at ≤16 weeks reduces pre-eclampsia risk by 52% (RR 0.48), compared to minimal benefit when started at ≥20 weeks (RR 0.82) 4
Use a dose of 75-162 mg daily (commonly 81 mg in North America), as this range provides optimal efficacy while minimizing bleeding risk 2, 6
Continue ASA throughout pregnancy until delivery, as discontinuation may eliminate protective effects 1
Clinical Benefits Beyond Pre-eclampsia
ASA reduces severe pre-eclampsia by 90% (RR 0.10) when started before 16 weeks in high-risk women 4
ASA decreases gestational hypertension by 69% (RR 0.31) in women with abnormal uterine artery Doppler 4
ASA reduces intrauterine growth restriction by 49% (RR 0.51) when initiated early in pregnancy 4
ASA lowers preterm birth rates by 20% (RR 0.80, NNT 37) across multiple studies 6
ASA reduces perinatal death by 21% (RR 0.79, NNT 167) in high-risk pregnancies 6
Safety Profile and Adverse Effects
Bleeding complications are rare, occurring in approximately 1 in 200 women (NNH 200), primarily involving minor bleeding during pregnancy 6
ASA prolongs maternal bleeding time but does not cause excessive blood loss during delivery or affect neonatal hemostasis 7
No increase in congenital abnormalities has been documented with low-dose ASA use during pregnancy 6
ASA does not affect neonatal platelet function when used at low doses, as maternal platelet inhibition does not transfer to the fetus 7
Management of ASA-Sensitive Pregnant Women
ASA desensitization is safe and effective for pregnant women with ASA hypersensitivity who require ASA for antiphospholipid syndrome or pre-eclampsia prevention 8, 3
Perform desensitization in an intensive care setting using incremental oral doses from 0.1 mg to 125 mg over 24 hours, with monitoring for hypersensitivity reactions 3
Antihistamines may be required during desensitization, but severe reactions are uncommon when properly monitored 3
Critical Pitfalls to Avoid
Do not delay ASA initiation beyond 16 weeks, as late initiation (≥20 weeks) provides minimal benefit for pre-eclampsia prevention (RR 0.82 vs 0.48 for early initiation) 4
Do not use ASA as monotherapy for antiphospholipid syndrome, as combination with low molecular weight heparin is required for optimal pregnancy outcomes 3
Do not withhold ASA due to concerns about bleeding, as the absolute risk increase is minimal (1 in 200) compared to substantial benefits 6
Do not confuse low-dose ASA with higher analgesic doses, as the therapeutic range for pregnancy prophylaxis is 75-162 mg daily, not higher doses used for pain management 2
Monitoring and Follow-up
Assess blood pressure at each prenatal visit to detect development of hypertensive disorders despite ASA prophylaxis 2
Monitor for proteinuria regularly in high-risk women, as ASA reduces but does not eliminate pre-eclampsia risk 1
Perform serial fetal growth assessments with ultrasound to detect intrauterine growth restriction, which remains possible despite ASA therapy 2
Continue ASA through delivery unless contraindications develop, as premature discontinuation may increase risk of late-onset complications 1