Should I Take Aspirin For Fertility? What The Evidence Actually Says
Low-dose aspirin is frequently discussed in fertility forums, recommended between friends and sometimes included in IVF or recurrent miscarriage treatment plans.
Because aspirin is inexpensive and readily available, it can be tempting to view “baby aspirin” as a harmless extra that might improve blood flow, support implantation or prevent miscarriage.
The reality is more nuanced.
Low-dose aspirin can be an important treatment for people with particular medical or obstetric indications. However, this does not mean that it improves fertility outcomes for everyone.
Research examining the routine use of aspirin during IVF has not shown a consistent improvement in pregnancy or live birth rates. Aspirin alone has also not been shown to reliably prevent otherwise unexplained miscarriage.
Aspirin is a medication that affects platelet function and bleeding. It may be unsuitable for people with a history of stomach ulcers, gastrointestinal bleeding, aspirin-sensitive asthma, bleeding disorders or certain other medical conditions.
Here, we examine when low-dose aspirin may be beneficial, when the evidence does not support its routine use, and why it should not be started during fertility treatment without appropriate medical advice.
What is low-dose aspirin?
Aspirin is a non-steroidal anti-inflammatory drug, or NSAID.
At higher doses, aspirin can be used to relieve pain and reduce inflammation. At lower doses, its principal effect is on platelets: the small blood cells involved in clot formation.
By reducing platelet aggregation, low-dose aspirin makes the blood less likely to form certain types of clot. This is why it is sometimes described as a “blood thinner”, although it does not literally make the blood thinner.
In the UK, low-dose aspirin generally refers to doses between 75mg and 150mg. The correct dose depends on why it is being prescribed and the protocol being followed.
The term “baby aspirin” is more commonly used in the United States. It can be misleading because it may give the impression that the medication is especially mild or harmless. It is still aspirin and still has potential risks and contraindications.
Does aspirin improve fertility?
Low-dose aspirin is not routinely recommended as a general treatment to improve fertility.
There are several theories about why aspirin could potentially help. These include reducing platelet activity, modifying inflammatory pathways and improving blood flow to the uterus or developing placenta.
However, a plausible biological mechanism does not necessarily translate into improved pregnancy or live birth rates.
Fertility is influenced by numerous factors, including egg and sperm quality, embryo development, chromosomal health, the uterine environment, hormonal signalling and the timing of implantation. It is rarely as simple as improving “blood flow” or making the blood less likely to clot.
For someone trying to conceive naturally without a diagnosed medical indication, there is currently insufficient evidence to recommend taking aspirin routinely.
Does aspirin improve implantation?
There is no good evidence that low-dose aspirin reliably improves implantation for the average fertility patient.
Implantation involves communication between the embryo and endometrium, controlled inflammation, vascular development and changes to the maternal immune system.
Because blood vessels and inflammatory signalling are involved, aspirin has been proposed as a possible treatment for implantation failure. However, this does not mean that implantation failure is usually caused by excessive clotting or inadequate blood flow.
Studies have not demonstrated a consistent improvement in implantation, clinical pregnancy or live birth rates when aspirin is given routinely to unselected fertility patients.
Should everyone undergoing IVF take aspirin?
No.
Aspirin is sometimes included in IVF protocols, but this does not mean it has been shown to benefit everyone undergoing treatment.
A Cochrane review found no evidence that routine aspirin improved pregnancy rates in a general IVF population. The American Society for Reproductive Medicine has also recommended against the routine use of low-dose aspirin to improve live birth rates in the general assisted reproduction population.
This does not mean that aspirin is never appropriate during IVF. A fertility specialist may prescribe it because of an individual’s diagnosis, medical history, previous pregnancy complications or a specific clinic protocol.
However, it should not be added independently simply because another patient has been prescribed it.
Why do some fertility clinics prescribe aspirin?
There are several possible reasons.
A patient may have a recognised medical indication, such as antiphospholipid syndrome, or an obstetric history that changes the balance of potential benefits and risks.
Some clinics may also include aspirin in individualised protocols because of concerns about placental function, platelet activity or previous treatment history.
In other cases, aspirin may be used because it has a plausible mechanism, is inexpensive and has been part of the clinic’s usual protocol for many years.
However, inclusion in a fertility protocol does not necessarily mean aspirin has been proven to improve live birth rates for every patient receiving it.
Different clinics may interpret limited or uncertain evidence differently. The most useful question is therefore not simply, “Does your clinic prescribe aspirin?” but:
What specific indication is aspirin intended to treat in my case?
Can aspirin prevent miscarriage?
Sometimes, but only in particular circumstances.
Aspirin is not a universal treatment for miscarriage and aspirin alone has not consistently been shown to prevent otherwise unexplained pregnancy loss.
Miscarriage can occur for many reasons. Chromosomal abnormalities within the embryo are the most common cause of sporadic early miscarriage. Other possible contributors include uterine abnormalities, hormonal or metabolic conditions, parental chromosomal rearrangements, sperm DNA fragmentation and antiphospholipid syndrome.
In many cases of recurrent miscarriage, no single cause is identified.
Taking aspirin does not correct most of these factors.
The clearest evidence for aspirin in recurrent miscarriage relates to people with confirmed antiphospholipid syndrome, where it is generally used alongside heparin rather than as a standalone treatment.
What is antiphospholipid syndrome?
Antiphospholipid syndrome, usually abbreviated to APS, is an autoimmune condition associated with an increased tendency to form blood clots and with certain pregnancy complications.
These can include recurrent miscarriage, later pregnancy loss, placental insufficiency, fetal growth restriction and pre-eclampsia.
For people with confirmed APS and recurrent miscarriage, treatment with low-dose aspirin and heparin injections during pregnancy can improve the likelihood of a successful pregnancy. RCOG patient guidance specifically recognises the use of this combination in women with APS and recurrent miscarriage.
Aspirin and heparin act differently. Aspirin primarily affects platelets, while heparin acts on other parts of the clotting system. They are not interchangeable.
It is also important to understand that one isolated positive antibody result does not necessarily establish a diagnosis of APS.
Diagnosis requires a relevant clinical history alongside specific blood-test findings. Antiphospholipid antibody tests usually need to remain positive when repeated at the appropriate interval.
Testing and treatment should therefore be overseen by a clinician experienced in recurrent miscarriage, haematology or maternal medicine.
Should I take aspirin after recurrent miscarriage?
Not automatically.
People with recurrent miscarriage should ideally receive appropriate investigation rather than being advised simply to try aspirin “just in case”.
For confirmed APS, aspirin combined with heparin may be recommended.
For unexplained recurrent miscarriage, however, evidence does not support routinely prescribing aspirin or heparin to everybody. ESHRE guidance distinguishes APS from unexplained recurrent pregnancy loss and does not support indiscriminate anticoagulant treatment where a relevant condition has not been identified.
Taking medication without an identified indication can create risks without addressing the actual reason for pregnancy loss.
What if I have an inherited thrombophilia?
An inherited thrombophilia is not the same as antiphospholipid syndrome.
Examples include factor V Leiden, prothrombin gene variants and deficiencies of certain natural anticoagulant proteins.
The relationship between inherited thrombophilia and early recurrent miscarriage is complex. A thrombophilia diagnosis does not automatically mean that aspirin is required.
Aspirin is also not a general-purpose treatment for every type of clotting risk.
Depending on the condition and personal history, a patient may require specialist monitoring, heparin, another treatment or no anticoagulant treatment at all.
These decisions should be made by an appropriate specialist rather than based on the results of a commercial clotting panel or advice from an online fertility forum.
Is aspirin useful for recurrent implantation failure?
There is insufficient evidence to recommend aspirin routinely for recurrent implantation failure.
The term recurrent implantation failure itself can include many different situations. These may involve embryo quality, embryo chromosome status, endometrial factors, uterine abnormalities, transfer technique and chance.
It should not automatically be interpreted as evidence of a clotting or blood-flow problem.
Some specialists may recommend aspirin within a carefully individualised protocol, but this should not be confused with strong evidence that it improves live birth rates for all patients with unsuccessful embryo transfers.
Is low-dose aspirin safe when trying to conceive?
Low-dose aspirin has an established role in several areas of reproductive and obstetric medicine when prescribed for an appropriate reason.
However, “commonly prescribed” does not mean “suitable for everybody”.
Aspirin irreversibly affects platelet function for the lifespan of the affected platelet. This means its effects on clotting continue after the tablet itself has left the bloodstream.
Potential adverse effects include indigestion, stomach irritation, bruising and bleeding. More serious gastrointestinal bleeding can occur, particularly in people with relevant risk factors or when aspirin is combined with other medicines that affect the stomach or clotting.
It should therefore be treated as a medicine, not as a fertility supplement.
Who should not take aspirin without medical advice?
You should speak to a doctor or pharmacist before taking low-dose aspirin if you:
- currently have, or have previously had, a stomach or duodenal ulcer;
- have experienced gastrointestinal bleeding;
- have an allergy to aspirin, ibuprofen or another NSAID;
- have asthma, particularly if aspirin or NSAIDs worsen your symptoms;
- have a diagnosed bleeding or platelet disorder;
- have significant liver or kidney disease;
- have unexplained bleeding during fertility treatment or pregnancy;
- are due to undergo egg collection, surgery or another invasive procedure;
- take anticoagulants or other antiplatelet medicines;
- regularly take ibuprofen, naproxen or another NSAID;
- or take another medicine that may interact with aspirin or increase bleeding risk.
The NHS advises that aspirin may not be suitable for people with a previous stomach ulcer, asthma, clotting problems, liver or kidney conditions, or previous reactions to aspirin and other NSAIDs.
This list is not exhaustive. Always disclose any supplements, over-the-counter medicines and prescribed medication when asking whether aspirin is appropriate.
Can I take aspirin if I have previously had a stomach ulcer?
A history of stomach or duodenal ulceration is an important reason to seek medical advice before taking aspirin.
Aspirin can irritate the stomach lining and increase the risk of gastrointestinal bleeding.
The individual level of risk depends on factors including whether the ulcer is active, what caused it, whether there has been previous bleeding, other medications being taken and whether stomach-protective treatment is required.
Do not assume that taking a low dose or taking aspirin with food removes this risk.
Can aspirin affect ovulation?
Prostaglandins are involved in follicular rupture and ovulation.
Medicines that strongly inhibit prostaglandin production, particularly some NSAIDs taken at anti-inflammatory doses, have been associated with delayed or disrupted follicular rupture in certain circumstances.
Low-dose aspirin is not identical to taking regular anti-inflammatory doses of ibuprofen, naproxen or aspirin. At low doses, aspirin’s most prominent effect is on platelet function.
There is no good evidence that prescribed low-dose aspirin routinely prevents ovulation. Nevertheless, its interaction with prostaglandin pathways is another reason not to add it casually when trying to conceive.
If aspirin reduces prostaglandins, could it interfere with implantation or labour?
Aspirin inhibits cyclooxygenase enzymes involved in producing prostaglandins and thromboxanes.
Prostaglandins have important roles throughout reproduction, including ovulation, implantation, cervical ripening and uterine contractions.
It therefore seems logical to ask whether taking aspirin could interfere with these processes.
The answer depends on the dose, timing, tissues affected and the clinical outcome being measured.
At low doses, aspirin predominantly inhibits platelet thromboxane production. It does not produce the same systemic anti-inflammatory effect as regularly taking aspirin at standard pain-relief doses.
Theoretical biological effects also do not necessarily translate into a meaningful increase in failed implantation, delayed labour or Caesarean birth.
Low-dose aspirin is routinely prescribed during pregnancy to people at increased risk of pre-eclampsia, and NICE advises 75mg to 150mg daily from 12 weeks until birth for those meeting specified risk criteria.
This is different from taking high-dose aspirin or other NSAIDs for pain during later pregnancy.
Anyone prescribed aspirin during pregnancy should follow the start and stopping instructions given by their maternity team rather than discontinuing it because of theoretical concerns about labour.
Why is aspirin prescribed to prevent pre-eclampsia?
Pre-eclampsia is a pregnancy complication involving abnormal placental development and maternal blood-vessel dysfunction.
For pregnant people at increased risk, low-dose aspirin can reduce the likelihood of developing pre-eclampsia and related placental complications.
NICE recommends 75mg to 150mg of aspirin daily from 12 weeks until birth for women at high risk of pre-eclampsia and for those with more than one moderate risk factor. Recognised high-risk factors include previous hypertensive disease in pregnancy, chronic kidney disease, autoimmune disease such as lupus or APS, diabetes and chronic hypertension.
This is an obstetric indication.
It does not mean aspirin improves egg quality, fertilisation or natural fertility, and it does not justify beginning aspirin before pregnancy without medical advice.
When should aspirin be started during fertility treatment?
There is no universal fertility start date for aspirin.
The timing depends entirely on why it is being used.
For example:
- An APS treatment plan may begin before conception or around confirmation of pregnancy, with heparin introduced according to the specialist protocol.
- A fertility clinic using aspirin during IVF may begin it at a particular stage of stimulation, embryo transfer or the luteal phase.
- Aspirin for pre-eclampsia prevention is generally started from 12 weeks of pregnancy under UK guidance.
- Other individual indications may have different timings.
Copying another person’s dose or start date is therefore not appropriate.
When should aspirin be stopped?
There is no single stopping point that applies to every fertility or pregnancy patient.
NICE guidance for pre-eclampsia prevention advises continuing aspirin until birth. Some local maternity protocols use a different stopping point, depending on dose, individual risk, planned procedures and local practice.
Patients undergoing fertility procedures may also be given specific instructions about aspirin before egg collection or surgery because of bleeding risk.
Do not independently stop aspirin that has been prescribed for APS, pre-eclampsia prevention or another medical indication. Equally, do not continue self-prescribed aspirin indefinitely without discussing it with the clinician responsible for your care.
Is aspirin the same as heparin?
No.
Although both may be described as “blood-thinning” treatments, they work in different ways.
Aspirin is an antiplatelet medicine. It reduces the ability of platelets to clump together.
Heparin is an anticoagulant. It acts on proteins within the clotting cascade and is usually given by injection.
In APS-related recurrent miscarriage, aspirin and heparin are commonly used together because they have complementary effects.
Aspirin should not be used as a substitute for prescribed heparin, and heparin should not be added without specialist assessment.
Is heparin the same as Clexane?
Not exactly. Clexane® is a brand name for enoxaparin, which is a type of low molecular weight heparin (LMWH).
Other LMWH brands include Inhixa®, Bemiparin (Zibor®) and Fragmin® (dalteparin).
Although patients often use the words "heparin" and "Clexane" interchangeably, Clexane is one specific type of heparin.
What should I ask my fertility clinic?
Before taking aspirin, ask:
- What specific diagnosis or risk factor are we treating?
- Is aspirin intended to improve fertility, reduce miscarriage risk or prevent a later pregnancy complication?
- What evidence supports aspirin in my particular situation?
- What dose should I take?
- When should I start it?
- When should I stop it?
- Should it be paused before egg collection or another procedure?
- Does it interact with my other medication?
- Do I have any history that increases my risk of bleeding or stomach ulceration?
- What symptoms should prompt me to seek medical advice?
A clear clinical rationale is more useful than being told that aspirin is simply “part of the protocol”.
Do I need Clexane if I'm taking aspirin?
AspirinClexaneTabletInjectionAntiplateletAnticoagulantMainly affects plateletsMainly affects clotting factorsUsed for some fertility and obstetric indicationsUsed for different indications including APS and thrombosis preventionThey are sometimes prescribed togetherThey are not interchangeable
The bottom line
Low-dose aspirin is not a universal fertility treatment.
It may form an important part of care for people with a specific diagnosis, particularly antiphospholipid syndrome, and it is also prescribed during pregnancy for some people at increased risk of pre-eclampsia.
However, routine aspirin has not been shown to increase live birth rates in the general IVF population. Aspirin alone does not reliably prevent otherwise unexplained miscarriage, and it should not be taken simply because it appears to be a low-risk addition to a fertility protocol.
Because aspirin affects platelet function and bleeding, it should be treated as medication rather than as a harmless supplement.
Before taking it, ask your fertility clinic, GP or specialist:
- what indication is being treated;
- what evidence supports its use in your situation;
- what dose is appropriate;
- when it should be started and stopped;
- and whether your medical history or other medication makes it unsuitable.
Never start or discontinue prescribed aspirin solely on the basis of information in this article.
This article provides general educational information and is not a substitute for personalised medical advice. Aspirin should only be used during fertility treatment or pregnancy following advice from an appropriately qualified healthcare professional.