Aspirin is one of the most well-studied medications in obstetric practice. At low doses, it has a well-established role in reducing the risk of one of the most serious complications of pregnancy pre-eclampsia. And yet, many women are surprised to be prescribed it, often because aspirin is something they associate with headaches or heart disease rather than antenatal care. This article explains the evidence behind its use, who should take it, when to start, and what it actually does.
What Is Pre-Eclampsia?
Pre-eclampsia is a pregnancy-specific condition characterised by high blood pressure and signs of organ involvement, most commonly affecting the kidneys, liver, or both. It typically develops after 20 weeks of pregnancy and, in its severe form, can be life-threatening for both mother and baby. It is also one of the leading causes of preterm birth, as early delivery is often the only definitive treatment.
The underlying problem in pre-eclampsia begins much earlier than the symptoms, in the first trimester, when the placenta is establishing itself. In women who go on to develop pre-eclampsia, this placentation process is impaired: the blood vessels that should remodel and open up to supply the growing placenta, are not able to do it adequately. The result is that a placenta remains under stress, and a cascade of effects that ultimately manifest as the clinical syndrome we recognise later in pregnancy.
How Does Aspirin Help?
Low-dose aspirin — typically 150mg taken at night — works primarily by inhibiting the production of a substance called thromboxane A2- a substance that promotes platelet stickiness and causes blood vessels to constrict. By reducing this effect, aspirin improves blood flow to the placenta and helps support the implantation and early vascular development that underpins a healthy pregnancy.
There is also evidence that aspirin has anti-inflammatory properties that may contribute to its protective effect in early pregnancy, though the vascular mechanism remains the most clinically important.
It is important to be clear: aspirin does not eliminate the risk of pre-eclampsia. But the evidence — and it is robust evidence, from large randomised controlled trials including the landmark ASPRE trial, shows that in high-risk women who start aspirin before 16 weeks of pregnancy, the risk of preterm pre-eclampsia (the most dangerous form, delivering before 37 weeks) is reduced by approximately 62%. That is a clinically meaningful reduction.
Who Should Take It?
Not every pregnant woman needs aspirin. It is recommended for women who are at increased risk of pre-eclampsia, based on their individual risk profile. Current guidance from NICE and RCOG uses a combination of factors to identify these women.
High-risk women — those in whom aspirin is clearly indicated — include those with:
A previous pregnancy complicated by pre-eclampsia
Chronic hypertension (high blood pressure diagnosed before pregnancy)
Chronic kidney disease
Diabetes — type 1 or type 2
Autoimmune conditions such as systemic lupus erythematosus (SLE) or antiphospholipid syndrome
Multiple pregnancy
Age over 40
Moderate-risk women — those in whom aspirin is recommended if two or more moderate risk factors are present, include those with a first pregnancy, BMI over 35, a family history of pre-eclampsia, or a pregnancy interval of more than ten years.
In addition to this clinical risk-factor approach, a more sophisticated method of screening is now available and widely used in specialist centres. First trimester combined screening, carried out between 11 and 13+6 weeks, incorporates the mother’s blood pressure, uterine artery Doppler measurements, a blood test for placental growth factor (PlGF), and maternal characteristics to generate a personalised risk calculation. This approach, developed by the Fetal Medicine Foundation, is more accurate than clinical risk factors alone and allows aspirin to be targeted at the women most likely to benefit.
When Should It Be Started?
This is one of the most important points, and one that is frequently missed in practice. Aspirin must be started before 16 weeks of pregnancy to be effective. Ideally, it should be started between 11 and 14 weeks, when first trimester screening is performed and the risk calculation is made.
Starting aspirin after 16 weeks does not confer the same benefit. The window of opportunity relates to the timing of placental development; once that critical early
vascular remodelling period has passed, aspirin cannot correct what has already been established. This is why early booking, early risk assessment, and prompt prescription matter so much.
How Is It Taken?
The recommended dose is 150mg at night. Night-time dosing is not arbitrary, studies suggest it results in better bioavailability and greater efficacy than morning dosing, likely related to circadian variation in platelet activity.
Aspirin should generally be continued until 36 weeks of pregnancy, at which point the risk of pre-eclampsia developing is low and the theoretical (though small) risk of bleeding around the time of delivery becomes a relevant consideration.
Women are sometimes concerned about taking aspirin because they associate it with bleeding risk. At the low doses used in pregnancy, the absolute risk of serious bleeding complications is very small, and this is far outweighed by the benefit in women who are genuinely high-risk. It is always worth discussing any concerns with your clinician, but for the right patient, the evidence firmly supports its use.
What Aspirin Cannot Do
It is worth being explicit about the limits of aspirin. It reduces the risk of preterm pre-eclampsia significantly, but it does not eliminate it. Women on aspirin still require careful monitoring throughout pregnancy, regular blood pressure checks, urine analysis, and growth scans where indicated. Aspirin is one part of a management strategy, not a substitute for vigilant antenatal care.
It also does not prevent all forms of pre-eclampsia. Term pre-eclampsia, developing close to or at 40 weeks, has a somewhat different pathophysiology and is less amenable to aspirin prophylaxis. This is why ongoing surveillance remains essential regardless.
For women with complex medical histories planning a pregnancy, preconception counselling is the ideal time to discuss aspirin prophylaxis as part of a broader antenatal risk management plan, so that it can be started promptly at the right gestational age.
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