The most studied protocol for baby aspirin while trying to conceive is 81 mg daily, started before conception and continued through early pregnancy. That dose and timing come from the EAGeR trial, a large randomized trial that tested preconception low-dose aspirin in women with a history of pregnancy loss. But who actually benefits, when to start, and how long to keep taking it are more nuanced than any single instruction can capture, and the answers depend heavily on your medical history.
The Standard Protocol From the Largest Trial
The Effects of Aspirin in Gestation and Reproduction (EAGeR) trial is the study most reproductive specialists reference when discussing baby aspirin and conception. It enrolled women aged 18 to 40 who had experienced one or two prior pregnancy losses and had no infertility diagnosis. Participants were randomized to take 81 mg of aspirin daily, along with folic acid, or a placebo with folic acid. Treatment began before conception and continued for up to six menstrual cycles of trying; women who became pregnant kept taking their assigned treatment through 36 weeks of gestation.1The Lancet. Preconception-initiated low-dose aspirin and pregnancy outcomes in women with prior pregnancy loss: a randomised controlled trial
So the practical protocol is straightforward: one baby aspirin (81 mg) every day, ideally taken at the same time, starting before you begin trying to conceive. You don’t need to time it to a particular point in your menstrual cycle. In the EAGeR trial, women simply took it daily from enrollment onward. A per-protocol analysis later found that adherence mattered: women who took their aspirin at least five out of seven days per week saw better outcomes than those who were less consistent, with 15 more live births and 6 fewer pregnancy losses per 100 women compared to placebo.2PubMed Central. The Effect of Preconception-Initiated Low-Dose Aspirin on Human Chorionic Gonadotropin-Detected Pregnancy, Pregnancy Loss, and Live Birth: Per Protocol Analysis of a Randomized Trial
Who Actually Benefits
The EAGeR trial’s overall results were more modest than many people realize. Across all participants, women taking aspirin had a live birth rate of 58% compared to 53% on placebo, a difference that did not reach statistical significance.1The Lancet. Preconception-initiated low-dose aspirin and pregnancy outcomes in women with prior pregnancy loss: a randomised controlled trial In other words, baby aspirin did not help everyone across the board. But a specific subgroup did see a clear benefit: women who had experienced a single pregnancy loss before 20 weeks within the previous year. In that group, aspirin increased the chance of getting pregnant by about 28% and significantly improved live birth rates.3PubMed Central. Preconception low dose aspirin and time to pregnancy: findings from the effects of aspirin in gestation and reproduction randomized trial
Women with two prior losses or losses later in pregnancy did not see the same benefit. The trial’s own conclusion stated that low-dose aspirin is not recommended as a general prevention for pregnancy loss. This matters because the internet often flattens the finding into “baby aspirin helps you get pregnant,” which overstates what the evidence showed. The clearest signal was in that narrower group of women with one recent early loss.
The Inflammation Factor
A secondary analysis of the EAGeR trial uncovered something that helps explain why aspirin works for some women and not others. Researchers measured high-sensitivity C-reactive protein (hsCRP), a marker of low-grade inflammation in the body, at baseline. Women with the highest levels of this inflammation marker had the worst pregnancy and live birth rates when they were on placebo: just 44% achieved a live birth. But when those same high-inflammation women took daily low-dose aspirin, their live birth rate climbed to 59%, a 35% relative increase. Women with lower inflammation levels showed no meaningful difference between aspirin and placebo.4PubMed Central. Preconception Low-Dose Aspirin Restores Diminished Pregnancy and Live Birth Rates in Women With Low-Grade Inflammation: A Secondary Analysis of a Randomized Trial
A related analysis looked at metabolic syndrome components alongside inflammation and found a consistent pattern: women with higher baseline C-reactive protein (at or above 2 mg/L) tended to benefit more from aspirin regardless of their metabolic status.5PubMed Central. Metabolic syndrome and the effectiveness of low-dose aspirin on reproductive outcomes This is worth knowing because most people trying to conceive do not routinely get their hsCRP checked. If you’ve had an early pregnancy loss and your doctor suspects chronic low-grade inflammation could be a factor, baby aspirin may be particularly worth discussing.
How Aspirin May Help With Conception
Aspirin’s potential fertility benefit appears to work through two main channels: improving blood flow to the uterus and dampening inflammation that can interfere with implantation. On the blood flow side, aspirin inhibits the production of thromboxane, a molecule that constricts blood vessels. By tipping the balance toward prostaglandins that relax blood vessel walls, aspirin can increase blood flow through the uterine and ovarian arteries.
In women with recurrent pregnancy loss, low-dose aspirin has been shown to significantly reduce resistance in uterine artery blood flow during the implantation window.6PubMed. Low-dose aspirin improves endometrial receptivity in the midluteal phase in unexplained recurrent pregnancy loss A meta-analysis of women who had intrauterine adhesions found that aspirin-based treatment was associated with thicker endometrial lining and better blood flow indices after surgical correction.7PubMed. The effect of aspirin on uterine arterial blood flow and endometrium in moderate and severe intrauterine adhesion after transcervical resection of adhesion: a systematic review and meta-analysis A thicker, better-perfused lining is generally considered more receptive to an embryo trying to implant.
One small but notable study looking specifically at when aspirin was used during the cycle found that aspirin taken during the implantation window was associated with roughly doubled fecundability, meaning cycles where aspirin was used around implantation were about twice as likely to result in pregnancy.8PubMed Central. Analgesic use at ovulation and implantation and human fertility That study was small, and a single finding like that needs replication before you build a strategy around it. But it fits the broader picture of aspirin improving conditions for implantation rather than, say, boosting ovulation itself.
What About IVF
If you’re undergoing in vitro fertilization, the story is different. A Cochrane systematic review that pooled data from multiple randomized controlled trials found no evidence that aspirin improved live birth rates or clinical pregnancy rates for women undergoing IVF. The review concluded there is no basis for routinely prescribing aspirin to improve pregnancy rates in a general IVF population.9PubMed Central. Aspirin for in vitro fertilisation A separate randomized double-blind trial confirmed this, showing a clinical pregnancy rate of 32% in the aspirin group versus 31% in the placebo group, essentially no difference.10Human Reproduction. Does low-dose aspirin improve pregnancy rate in IVF/ICSI? A randomized double-blind placebo controlled trial
There is one older trial from 1999 that did find significantly higher pregnancy and implantation rates with low-dose aspirin in IVF patients, along with better ovarian responsiveness and improved blood flow measurements.11PubMed. Low-dose aspirin treatment improves ovarian responsiveness, uterine and ovarian blood flow velocity, implantation, and pregnancy rates in patients undergoing in vitro fertilization: a prospective, randomized, double-blind placebo-controlled assay However, larger and more recent evidence has not replicated that finding for a general IVF population. Some IVF clinics still prescribe baby aspirin on a case-by-case basis, particularly for patients with thin linings or blood flow concerns, but the blanket recommendation is gone. If your fertility clinic tells you to take it, follow their guidance for your specific protocol; if they don’t mention it, the evidence does not support adding it on your own.
Antiphospholipid Syndrome and Recurrent Loss
One scenario where aspirin is widely prescribed during conception is antiphospholipid syndrome (APS), an autoimmune condition where the body produces antibodies that increase the risk of blood clots. APS is one of the most treatable causes of recurrent pregnancy loss, and the standard treatment involves aspirin combined with a blood thinner, typically low-molecular-weight heparin (LMWH). A meta-analysis found that the combination of aspirin plus LMWH produced higher live birth rates than aspirin alone in women with APS, with a relative risk of 1.29 favoring the combination.12PubMed Central. Meta-analysis on aspirin combined with low-molecular-weight heparin for improving the live birth rate in patients with antiphospholipid syndrome and its correlation with d-dimer levels A separate review of five trials also confirmed this, finding a pooled relative risk of 1.27 in favor of heparin plus aspirin over aspirin alone.13Journal of Thrombosis and Haemostasis. Antithrombotic therapy to prevent recurrent pregnancy loss in antiphospholipid syndrome—What is the evidence?
In one smaller study, women with APS who took 75 mg of aspirin daily combined with LMWH achieved a live birth rate of 86%.14International Journal of Reproduction, Contraception, Obstetrics and Gynecology. The effect of low dose aspirin and low molecular weight heparin (enoxaparin) in recurrent pregnancy loss associated with antiphospholipid antibody syndrome If you’ve been diagnosed with APS, aspirin is not optional or speculative; it is a core part of standard treatment. The dose and whether you need heparin alongside it should be determined by your doctor based on your antibody levels and loss history.
Safety and Side Effects
The EAGeR trial provides the most relevant safety data for preconception aspirin use. Most women in the trial reported at least one symptom during the study period, but the rate was essentially identical between the aspirin and placebo groups: 74% versus 73%. The most common complaints were gastrointestinal discomfort and unusual bleeding, and these also did not differ between groups. The one statistically significant difference was vaginal bleeding during pregnancy, which was more common in the aspirin group (22% versus 17%).15PubMed Central. Complications and Safety of Preconception Low-Dose Aspirin Among Women With Prior Pregnancy Losses
That vaginal bleeding finding is worth understanding in context. At 81 mg, aspirin has a mild blood-thinning effect, so it is not surprising that some spotting increases. The trial found no increase in serious maternal complications like hemorrhage or placental problems. Still, if you notice new or increased vaginal bleeding while taking baby aspirin, let your provider know. It does not necessarily mean something is wrong, but it should be tracked.
You should avoid baby aspirin if you have an aspirin allergy, a bleeding disorder, or active peptic ulcer disease. NSAIDs like ibuprofen can interact with aspirin’s antiplatelet effects, so if you regularly take ibuprofen for pain, mention this to your doctor. Taking aspirin on a full stomach or with food can reduce gastrointestinal irritation.
When to Start and How Long to Continue
The EAGeR protocol started aspirin before conception and continued through 36 weeks of pregnancy. If your doctor is prescribing aspirin specifically for fertility purposes, expect to start it during the cycle you begin trying, not mid-pregnancy. There is no evidence that starting it after a positive pregnancy test captures the same benefit. The preconception start matters because the implantation window is the critical period for the blood flow and inflammation effects to be in place.
If you become pregnant while taking baby aspirin and your doctor transitions you to a preeclampsia prevention protocol, the timing guidance shifts. Research suggests that aspirin for preeclampsia prevention is most effective when started before 16 weeks of gestation, with doses above 100 mg daily appearing more protective than the lower doses recommended by some organizations.16PubMed. Low-dose aspirin therapy for the prevention of preeclampsia: time to reconsider our recommendations? A trial in high-risk women found that starting 75 mg of aspirin before 12 weeks of gestation reduced preeclampsia rates from about 29% in the control group to roughly 9% in the aspirin group.17Scientific Reports. Early initiation of low-dose aspirin for the prevention of pre-eclampsia in high-risk pregnancies If you are already on baby aspirin from trying to conceive, you have a head start on that timing window.
As for when to stop, most guidelines and trial protocols discontinue aspirin between 36 and 37 weeks to reduce bleeding risk during delivery. A randomized trial tested whether it was safe to stop aspirin earlier, between 24 and 28 weeks, in women whose blood markers suggested low preeclampsia risk at that point. That trial found stopping early was noninferior to continuing, meaning outcomes were not worse.18PubMed Central. Aspirin Discontinuation at 24 to 28 Weeks’ Gestation in Pregnancies at High Risk of Preterm Preeclampsia: A Randomized Clinical Trial But that decision should be made with your obstetrician based on your individual risk profile, not on your own.
Aspirin Resistance
Some people do not respond normally to aspirin. Their platelets remain active despite taking the drug, a phenomenon sometimes called aspirin resistance. A study of pregnant women on aspirin found that those with biochemical markers of aspirin resistance had significantly higher rates of preeclampsia, preterm delivery, and small-for-gestational-age babies.19PubMed. Aspirin resistance may be associated with adverse pregnancy outcomes Aspirin-resistant women in that study delivered about two weeks earlier on average (roughly 36 versus 38 weeks).
Aspirin resistance is not routinely tested for in preconception care, and there is no widely adopted protocol for what to do when it is identified. But it is a plausible explanation for why some women do not see the expected benefits from baby aspirin. If you have been taking aspirin faithfully and your doctor suspects it is not having the expected effect on your platelet function or blood flow measurements, testing for aspirin resistance is an option worth raising.
Should Your Male Partner Take Aspirin Too
This is a question that comes up and the short answer is probably not. A review of the available research concluded that aspirin appears to have negative effects on semen quality, potentially through reduced testosterone synthesis, decreased testicular prostaglandins, and increased oxidative damage to sperm.20PubMed. Effect of aspirin on semen quality: A review An animal study found that sustained aspirin use reduced sperm density, count, and motility in rats, with effects worsening over 30 and 60 days of treatment.21PubMed Central. Adverse Effects of Subchronic Dose of Aspirin on Reproductive Profile of Male Rats
That said, a systematic review noted that the overall evidence on aspirin and male fertility is still unclear, in part because there are very few clinical studies that directly examine the effect in men at typical low doses.22PubMed Central. Effects of major analgesics on male fertility: A systematic literature review The mechanistic concerns come largely from animal data and in vitro studies, not from trials of men trying to conceive. Still, if your partner is taking daily aspirin for another reason, like cardiovascular prevention, and you are having trouble conceiving, it is worth mentioning to a fertility specialist. There is no reason for a male partner to start taking aspirin in hopes of improving your chances of pregnancy.
What Your Doctor Needs to Know Before You Start
Baby aspirin is available over the counter and feels like a low-stakes decision, but starting it specifically to improve your chances of conceiving is worth a conversation with your provider. They will want to know your pregnancy loss history (if any), whether you have been tested for antiphospholipid antibodies or clotting disorders, and whether you take any medications that could interact with aspirin. If you have risk factors for preeclampsia, starting aspirin preconceptionally may serve double duty, addressing both implantation support and early preeclampsia prevention.
The evidence is strongest for women with a recent single early pregnancy loss. It is also well established for women with antiphospholipid syndrome, where aspirin is part of a combination protocol. For women without any history of loss or specific risk factors, the data does not support routine use. Daily adherence matters: the per-protocol analysis from EAGeR showed that taking it most days of the week, not just occasionally, was what drove the benefit. If you do start, take it consistently, with food if it bothers your stomach, and plan to continue through early pregnancy unless your doctor says otherwise.