When to start aspirin (acetylsalicylic acid) in pregnancy for preeclampsia prevention?

From the Guidelines

Low-dose aspirin (81-100 mg daily) should be started between 12-16 weeks of pregnancy and continued until delivery for women at high risk of preeclampsia. High-risk factors include previous preeclampsia, chronic hypertension, pre-gestational diabetes, renal disease, autoimmune conditions like lupus or antiphospholipid syndrome, and multiple gestation. Women with more than one moderate risk factor (first pregnancy, obesity, maternal age >35 years, family history of preeclampsia, or sociodemographic characteristics) should also receive prophylaxis. Aspirin works by inhibiting thromboxane production while preserving prostacyclin, improving the balance between these vasoactive compounds and promoting better placental development and blood flow. This reduces the risk of preeclampsia by approximately 10-15% 1. The medication should be taken at bedtime for optimal effect, as it aligns with the circadian rhythm of blood pressure. If started after 16 weeks, aspirin still offers some benefit but is less effective than when initiated earlier. The low dose minimizes bleeding risks while providing cardiovascular protection.

Some key points to consider:

• The American Congress of Obstetricians and Gynecologists recommends initiating use of low-dose aspirin (60 to 80 mg/d) during the late first trimester to prevent preeclampsia in women with a medical history of early-onset preeclampsia and preterm delivery (<34 weeks) or history of preeclampsia in more than 1 previous pregnancy 1.
• The World Health Organization recommends the use of low-dose aspirin (75 mg/d) starting as early as 12 to 20 weeks of gestation for high-risk women 1.
• The National Institute for Health and Care Excellence recommends that women at high risk for preeclampsia take 75 mg/d of aspirin from 12 weeks until delivery 1.
• The American Heart Association and the American Stroke Association recommend that women with chronic primary or secondary hypertension or previous pregnancy-related hypertension take low-dose aspirin from 12 weeks until delivery 1.
• The American Academy of Family Physicians recommends low-dose aspirin (81 mg/d) after 12 weeks of gestation in women who are at high risk for preeclampsia 1.

Overall, the evidence suggests that low-dose aspirin should be started between 12-16 weeks of pregnancy and continued until delivery for women at high risk of preeclampsia.

From the FDA Drug Label

If pregnant or breast-feeding ask a health professional before use. it is especially important to use aspirin during the last 3 months of pregnancy unless definitely directed to do so by a doctor because it my cause problems in the unborn child or complications during delivery. The FDA drug label does not answer the question.

From the Research

Aspirin Initiation for Preeclampsia Prevention

• The American College of Obstetricians and Gynecologists recommends initiating daily low-dose aspirin (81 mg/day) between 12 weeks and 28 weeks of gestation, optimally before 16 weeks, for women at high risk of preeclampsia 2.
• The World Health Organization recommends initiating 75 mg of aspirin before 20 weeks of gestation for women at high risk of preeclampsia 3.
• The Royal College of Obstetricians and Gynaecologists recommends 150 mg of aspirin daily from 12 weeks of gestation for women at increased risk of preeclampsia 3.
• A study found that doses of >100 mg of aspirin daily initiated before 16 weeks' gestation seem to be most effective at reducing the risk of preeclampsia 3.

Factors Influencing Aspirin Initiation

• Women with a history of early-onset preeclampsia and preterm delivery at less than 34 0/7 weeks of gestation, or those with more than one prior pregnancy complicated by preeclampsia, should start taking low-dose aspirin in the late first trimester 2.
• Women with more than one moderate risk factor for preeclampsia, such as first pregnancy, maternal age of 35 years or older, or a body mass index greater than 30, should consider taking low-dose aspirin 2.
• A study found that patients with chronic hypertension, a history of preeclampsia or gestational hypertension in a previous pregnancy, and pregestational diabetes mellitus were most likely to report receiving aspirin recommendation 4.

Dosage and Timing

• A randomized controlled study found that 150 mg of aspirin daily was more effective than 75 mg daily in preventing preeclampsia in high-risk pregnant women, with initiation between 12 to 16 weeks of gestation 5.
• A review of the literature suggests that both the dosage and timing of aspirin initiation are key to its effectiveness at reducing the risk of preeclampsia, with doses of >100 mg daily initiated before 16 weeks' gestation being most effective 3.
• The International Federation of Gynecology and Obstetrics initiative on preeclampsia recommends 150 mg of aspirin to be initiated at 11 to 14+6 week's gestation 3.