Many fertility clinics prescribe baby aspirin (typically 81 mg) during IVF cycles with the goal of improving blood flow to the uterus and supporting embryo implantation. However, the evidence behind this practice is more nuanced than you might expect. The American Society for Reproductive Medicine (ASRM) has actually recommended against routine use of low-dose aspirin to improve live birth rates in the general IVF population, rating this a Grade A recommendation based on high-quality studies. So why do so many clinics still prescribe it? The answer depends on your specific medical profile.
How Aspirin Works in the Uterus
Aspirin blocks platelet clumping and shifts the balance between two chemical signals in your blood vessels. One of those signals constricts blood vessels, while the other relaxes them. By tipping the balance toward the vessel-relaxing signal, aspirin increases blood flow to the uterine lining. The theory is straightforward: better blood flow means a more receptive environment for an embryo to implant.
There’s also evidence that aspirin increases levels of certain proteins in the endometrial lining that play a role in helping an embryo attach. This second mechanism is why some reproductive endocrinologists find the biological rationale compelling, even when the clinical trial results are mixed.
What the Evidence Actually Shows
Despite the logical mechanism, studies have not consistently shown that baby aspirin improves IVF success rates for the average patient. A Cochrane review of the available research found no clear benefit for the general IVF population. One large study found that aspirin did not enhance endometrial thickness, improve the ovarian response to stimulation drugs, or increase pregnancy rates compared to no aspirin at all.
The results across studies are contradictory in ways that make a firm conclusion difficult. One trial using 75 mg found a modest increase in clinical pregnancy rates (about 18% higher odds), while another using 100 mg found no benefit whatsoever. A third study, also at 100 mg, found a 52% increase in pregnancy rates compared to placebo. These inconsistencies likely reflect differences in patient populations, timing, and study design rather than meaningful dose differences. Because aspirin is inexpensive and carries relatively low risk, many clinics continue prescribing it on a “can’t hurt, might help” basis, even though professional guidelines don’t support routine use.
When Aspirin Has a Clearer Purpose
The case for baby aspirin becomes much stronger if you have specific risk factors. Your doctor may have a particular reason for prescribing it based on your medical history.
- Antiphospholipid syndrome or clotting disorders: Women with these conditions have historically been prescribed aspirin (sometimes combined with a blood thinner) to reduce miscarriage risk. If you have a known thrombophilia or have experienced recurrent pregnancy loss, aspirin addresses a real physiological problem rather than serving as a general booster.
- Pre-eclampsia prevention: IVF pregnancies carry a higher baseline risk of pre-eclampsia. ACOG recommends 81 mg daily for women with high-risk factors, including autoimmune disease, kidney disease, diabetes, chronic high blood pressure, or a history of pre-eclampsia. Moderate-risk factors like being 35 or older, having a BMI over 30, or carrying a first pregnancy also qualify if more than one applies. For pre-eclampsia prevention, the recommendation is to start between 12 and 16 weeks of pregnancy and continue until delivery.
- Multifetal gestation: If your IVF cycle results in twins or more, that alone is considered a high-risk factor for pre-eclampsia, making aspirin a clearer recommendation.
Typical Timing and Dosage
Protocols vary between clinics, which is part of why research results are so inconsistent. Some clinics start aspirin at the beginning of ovarian stimulation, others on the day of embryo transfer, and still others only after a positive pregnancy test. In frozen embryo transfer cycles, it may be started during the preparation phase before transfer. The standard dose across nearly all IVF protocols is 75 to 100 mg daily, with 81 mg (the standard baby aspirin sold in the U.S.) being the most common.
When aspirin is prescribed specifically for pre-eclampsia prevention rather than implantation support, timing shifts to later in the process. ACOG recommends starting between 12 and 28 weeks of gestation, ideally before 16 weeks, and continuing daily until delivery.
Bleeding Risk Around Egg Retrieval
Egg retrieval involves passing a needle through the vaginal wall into the ovaries, so any medication that reduces blood clotting deserves consideration. For most women on baby aspirin alone, the bleeding risk is minimal. However, if you’re also taking blood thinners for a clotting disorder or cardiac condition, your fertility team should coordinate with a hematologist to create a bridging plan. This balances the bleeding risk during the procedure against the clotting risk from your underlying condition, which is actually amplified by the high estrogen levels that IVF stimulation drugs produce.
Some clinics ask patients to stop aspirin a few days before retrieval and resume afterward. Others continue it straight through. If your clinic hasn’t given you clear instructions on this, ask specifically about the retrieval window.
Why Your Clinic Might Prescribe It Anyway
Given that major professional organizations recommend against routine use, it’s reasonable to wonder why your doctor prescribed it. A few factors explain the disconnect. Aspirin costs pennies per day and has a well-established safety profile at low doses. The potential biological mechanisms are plausible. And for patients investing thousands of dollars in an IVF cycle, both doctors and patients tend to favor interventions that might offer even a small edge, especially when the downside is minimal.
If your doctor prescribed baby aspirin and you don’t have any of the specific risk factors listed above, you’re likely getting it as a low-risk addition to your protocol rather than a targeted treatment for a diagnosed condition. That doesn’t mean it’s wrong to take it. It means the honest answer is that the evidence is uncertain, and your clinic has made a judgment call that the potential upside outweighs the negligible risk.