In high-risk pregnancies, daily low-dose aspirin can significantly reduce the most dangerous form of preeclampsia. So, for whom, at what dose and when should this recommendation be started? We have collected current evidence in plain language.
Preeclampsia (a serious pregnancy disease that causes pregnancy-related high blood pressure and affects organs such as the kidneys and liver) is one of the most important problems threatening the health of mothers and babies worldwide. The good news is that low-dose aspirin, a simple, inexpensive drug that has been used for years by some pregnant women, actually works in reducing the incidence of this disease. In this article, we will discuss what current scientific evidence says; We discuss who should use aspirin, when and in what dosage.
Preeclampsia usually occurs after the 20th week of pregnancy. The most typical symptom is increased blood pressure; This may be accompanied by protein leakage in the urine (proteinuria) or symptoms of damage to organs such as the liver, kidneys and brain. If left untreated, eclampsia (seizure/seizure), stroke and organ damage for the mother; For the baby, it can lead to consequences such as premature birth and developmental delay.
Especially the early form of the disease—that is, preterm preeclampsia, which requires delivery before the 37th week—is the most risky form for the mother and the baby. We will see that the strongest benefit of aspirin is precisely in this early form.
The basis of preeclampsia lies in the failure of the placenta (the organ that nourishes the baby) to settle well enough in the mother's vessels and subsequently an imbalance in the vessels. While the production of a substance called prostacyclin, which relaxes the vessels, decreases in the body, the production of a substance called thromboxane, which contracts the vessels and increases clotting, increases. Low-dose aspirin corrects this balance by suppressing thromboxane and helps the blood flow to the placenta be healthier. It is important to have a "low dose" here: unlike the high doses used as painkillers, the aim during pregnancy is not to relieve pain, but to support this vascular balance.
One of the best-known and most powerful studies in this field is the ASPRE study (Rolnik et al., New England Journal of Medicine, 2017). 1,776 singleton pregnant women at high risk for preterm preeclampsia were included in this multicenter, double-blind, placebo-controlled randomized study. Participants received either aspirin 150 mg or placebo (an inactive tablet) every night from 11–14 weeks of pregnancy until 36 weeks.
The result was striking: the incidence of preterm preeclampsia was 1.6% in the aspirin group, while it was 4.3% in the placebo group. This difference corresponds to an approximately 62% reduction in the risk of preterm preeclampsia (odds ratio/OR 0.38; 95% confidence interval 0.20-0.74). There was no significant difference between the groups in terms of side effects in newborns; So, at this dose, aspirin was found to be safe.
Recommendations from major healthcare organizations are broadly consistent; there is only a regional difference in the preferred dose.
In its recommendation updated in 2021, the United States Preventive Services Task Force (USPSTF) recommends 81 mg of low-dose aspirin daily after week 12 for pregnant women at high risk for preeclampsia, with a grade B (recommended) recommendation. Similarly, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) recommend starting 81 mg of aspirin daily between weeks 12 and 28, ideally before week 16, and continuing until birth. The 81 mg dose is preferred in the United States because it is the only dose sold there under the low-dose label.
The ASPRE study and many European-based protocols based on it use the 150 mg dose. Which dose is superior is an issue that is still being researched and does not have a clear answer. A recent systematic review and meta-analysis in 2025 (Wang and Chang) combining 28 studies reported that low-dose aspirin significantly reduces preterm preeclampsia when started within the first 16 weeks (relative risk 0.52; 95% CI 0.31-0.88) and that doses of 100 mg and above appear to be more effective than doses below 100 mg in reducing preterm preeclampsia. In contrast, a 2024 network meta-analysis (Hu et al) evaluating 23 randomized studies (10,547 pregnant women) highlighted the 80-100 mg/day range as the optimal dose in terms of balance of efficacy and side effects. As can be seen, current evidence does not fully converge on a single "best dose"; Both doses are in the low dose class and the choice is made individually.
Aspirin is not recommended for all pregnant women, but for those with an increased risk of preeclampsia. Your physician makes the risk assessment by taking the following factors into consideration.
Timing is important. The best protective effect of aspirin is achieved when it is started early, while placental implantation and development are still underway. Therefore, organizations recommend starting after week 12 and ideally before week 16. An analysis pooling 45 randomized trials showed that the benefit was more limited when aspirin was started after week 16. Your risk should therefore be assessed in early pregnancy and treatment should not be delayed when indicated.
Aspirin is usually taken at night and continued until close to birth (usually around the 36th week or until birth) according to your doctor's recommendation. Your doctor decides when to quit.
The low doses of aspirin used in this indication are generally considered safe during pregnancy. In both the ASPRE study and large reviews, no significant increase in harm to the mother or baby has been demonstrated at these doses. However, like any drug, there are situations in which aspirin is not suitable: it is not used in cases such as aspirin allergy, active gastrointestinal bleeding or some bleeding disorders. Therefore, the decision is always made individually.
Low-dose aspirin is an evidence-based and safe preventive approach that can significantly reduce early preeclampsia, the most dangerous form of preeclampsia, when used in the right pregnant woman and at the right time. Key points; Early assessment of the risk, starting the treatment before the 16th week, and your doctor deciding on the dose and duration. This article serves a general informational purpose; You will make the best decision for your own situation together with your doctor who follows you.
Last updated: . This content has undergone scientific verification and is not a substitute for a medical examination.