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#349 An ASA a day when a baby’s on the way?

Is acetylsalicylic acid (ASA) effective in preventing complications in pregnant women at risk of preeclampsia?

In women at risk for preeclampsia at ~12-28 weeks gestation, low-dose ASA (50-150mg) reduces risk of preeclampsia by an absolute ~2%, perinatal death by ~0.5%, and preterm birth by ~2% compared to placebo. The risk of postpartum hemorrhage is increased by up to ~1%.

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  • 7 systematic reviews (17-77 randomized controlled trials [RCTs]; 26,952-46,568 patients) from the last 5 years comparing ASA to placebo in pregnant women at varying preeclampsia risk.1-7 ASA usually initiated ~12-28 weeks, continued until delivery. Results statistically significant unless indicated.
    • Maternal Outcomes:
      • Preeclampsia: 5 systematic reviews (16-60 RCTs):1-5
        • 5-9.6% versus 5.8-11.8% (placebo), number needed to treat (NNT)=31-72.
      • Placental abruption: 3 systematic reviews (9-29 RCTs):1,3,4
        • 0.9-1.3% versus 0.7-1.2% (placebo) (not statistically different).
      • Postpartum hemorrhage (>500-1000mL blood loss): 4 systematic reviews (9-19 RCTs):1,3,4,6
        • 7-15.2% versus 3.3-14.3% (placebo), number needed to harm (NNH)=97-239 (1/4 systematic reviews not statistically different).4
    • Fetal outcomes:
      • Perinatal death: 3 systematic reviews (11-52 RCTs):1,3,4
        • 1-3.1% versus 2.7-3.5% (placebo), NNT=179-239.
      • Preterm delivery/birth: 2 systematic reviews with comprehensive data (18-47 RCTs):1,3
        • 0.9-16.6% versus 17.5-18.5% (placebo), NNT=54-64.
      • Fetal intracranial bleed: 1 systematic review (6 RCTs):4
        • Not statistically different.
    • Limitations: Inconsistent definitions of patients at risk for preeclampsia; infrequent reporting of serious maternal outcomes (examples: eclampsia, death); some large RCTs not included in all systematic reviews.

  • No clear difference in outcomes between 50-150 mg daily.1,3-5,7
  • Earlier initiation (<16-20 weeks) may enhance preeclampsia benefit based on subgroup analyses. No consistent trends for other outcomes.1-4,7
  • Sensitivity of clinical risk factors for predicting pre-eclampsia is <40%.8
  • Guidelines vary:
    • Common recommendations among guidelines for ASA use include, but not limited to:
      • Any high-risk factors (examples: prior preeclampsia, chronic hypertension, renal or autoimmune disease, diabetes) or,
      • At least 2 moderate-risk factors (examples: nulliparity, age >35-40, previous adverse pregnancy outcome).
    • Canadian: ASA 81-162mg daily preferably before 16 weeks until 36 weeks gestation.8
    • American: ASA 81mg daily initiated between 12-28 weeks gestation (optimally before 16 weeks) until delivery.9

William Garnet Warrian October 2, 2023

Helpful knowledge

Vanessa Montagliani October 2, 2023

Great clarification

Karen Tanaka October 2, 2023


Karen Tanaka October 2, 2023


Jaclyn Clements October 2, 2023

very helpful

Kimberly Warwick October 2, 2023

Gave me insight into the variety of doses we have been hearing about as well as duration. Have been initiating ASA ,when risk factors, btw 13 and 16 wks. We have listed obesity as a risk factor.

Andrew Affleck October 2, 2023

NNT high

Gloria Meneses October 2, 2023

This is good info and I will surely remember this when I see potential patients.

Parampal Sandhu October 3, 2023

this is new to me

Alexander Arthur October 7, 2023

Useful information

ALMAS JIWANI October 8, 2023

good learning point

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  • Brianne Desrochers PharmD candidate
  • Sasha Katwaroo PharmD candidate
  • Karen Toews MD CCFP
  • Jamie Falk PharmD

1. Duley L, Meher S, Hunter KE, et al. Cochrane Database Syst Rev. 2019: CD004659.

2. Wang Y, Guo X, Obore N, et al. Front Cardiovasc Med. 2022; 9:936560.

3. Choi YJ, Shin S. Am J Prev Med. 2021; 61(1):e31-e45.

4. Henderson JT, Vesco KK, Senger CA, et al. JAMA. 2021; 326(12):1192-1206.

5. van Doorn R, Mukhtarova N, Flyke IP, et al. PLoS One. 2021; 16(3):e0247782.

6. Jiang Y, Chen Z, Chen Y, et al. Am J Obstet Gynecol MFM 2023; 5:100878.

7. Turner JM, Robertson NT, Hartel G, et al. Ultrasound Obstet Gynecol. 2020; 55(2):157-169.

8. Magee LA, Smith GN, Bloch C, et al. J Obstet Gynaecol Can. 2022; 44(5):547–571.e1.

9. ACOG Committee. Obstet Gynecol. 2018; 132(1):254-256.

Authors do not have any conflicts of interest to declare.