When to Start Taking Baby Aspirin When TTC

The best-studied timing for starting low-dose aspirin while trying to conceive is before conception, not after a positive pregnancy test. The largest randomized trial on this topic, known as EAGeR, enrolled over 1,200 women and found that those who began 81 mg aspirin daily before becoming pregnant saw modestly better outcomes than those on placebo, while a per-protocol analysis showed that starting aspirin only after conception blunted the benefit. That said, the overall results of the trial were not statistically significant for all participants, and whether baby aspirin helps you specifically depends on factors like your history, inflammation levels, and metabolic health.

What the Largest Trial Actually Found

The EAGeR trial (Effects of Aspirin in Gestation and Reproduction) is the study most fertility specialists point to when the question of preconception aspirin comes up. It enrolled women aged 18 to 40 who had experienced one or two prior pregnancy losses and randomized them to either 81 mg aspirin daily or placebo, starting before they tried to conceive. Overall, about 58% of women on aspirin had live births compared with 53% on placebo. That five-percentage-point difference did not reach statistical significance for the full group.¹

But the trial had two strata of participants. Among women with only one prior loss (the “original stratum”), results were more encouraging: 62% on aspirin achieved a live birth versus 53% on placebo, a difference that was statistically significant. Among women with one or two losses (the “expanded stratum”), the benefit largely disappeared, with live birth rates around 54% and 52%.¹ When researchers looked at time to pregnancy, women in the original stratum who took aspirin conceived faster than those on placebo, with a roughly 28% increase in cycle-by-cycle odds of getting pregnant.²

Why Preconception Timing Matters

A per-protocol analysis of the EAGeR data specifically compared women who started aspirin before conception with those who started it only after becoming pregnant. The result was clear: beginning aspirin after conception reduced the estimated benefits relative to preconception use.³ The thinking behind this is straightforward: aspirin’s effects on blood flow, inflammation, and the uterine lining take time to establish. If the goal is to support implantation and the earliest stages of placental development, the drug needs to be on board before the embryo arrives. Starting at the point of a positive test means you’ve already missed the implantation window.

In practical terms, this means that if your provider recommends baby aspirin for fertility reasons, the standard advice drawn from this trial is to start it while you are actively trying, not to wait and see whether you conceive first. Most women in the EAGeR trial began taking aspirin at the start of their menstrual cycle and continued daily through pregnancy.

Who Benefits Most

The overall EAGeR results were modest, but secondary analyses have identified subgroups where aspirin made a much bigger difference. The clearest beneficiaries appear to be women with low-grade systemic inflammation. Researchers measured high-sensitivity C-reactive protein (hsCRP), a blood marker of inflammation, and found that women in the highest third of hsCRP values had a 44% live birth rate on placebo but a 59% live birth rate on aspirin, amounting to a 35% relative increase. Women with lower inflammation levels showed no meaningful difference between aspirin and placebo.⁴ In other words, aspirin seemed to rescue the fertility-dampening effect of chronic low-level inflammation, bringing those women’s outcomes in line with everyone else’s.

A separate analysis from the same trial examined metabolic health. Among women with no components of metabolic syndrome (conditions like elevated blood sugar, high blood pressure, or abdominal obesity), aspirin was associated with roughly 14 more live births per 100 couples compared with placebo. As the number of metabolic syndrome components went up, the benefit shrank and eventually disappeared.⁵ This is a somewhat counterintuitive finding. You might expect women who are metabolically unhealthy, who already have more inflammation, to benefit most. Instead, it appears that metabolic syndrome creates enough physiological disruption that aspirin alone can’t overcome it.

How do these two findings square with each other? One possibility is that the inflammation responsive to aspirin is the kind that exists in otherwise healthy women, perhaps related to immune activity at the uterine lining, rather than the widespread metabolic inflammation linked to insulin resistance and obesity. The takeaway for someone TTC: if you’re metabolically healthy but have had a prior early pregnancy loss, the data is most favorable for you. If you have multiple features of metabolic syndrome, aspirin alone is unlikely to move the needle much.

When Aspirin Does Not Seem to Help

Despite the hopeful subgroup findings, there are scenarios where baby aspirin hasn’t shown a clear benefit, and it’s worth knowing about them before assuming the pill is a universal fertility booster.

A 2025 systematic review and meta-analysis pooled three randomized trials in women with unexplained recurrent pregnancy loss and found no significant improvement in live birth rates with daily low-dose aspirin compared with no treatment.⁶ This is an important distinction. The EAGeR trial enrolled women with only one or two losses, many of whom may have lost pregnancies for random chromosomal reasons. Recurrent pregnancy loss (typically defined as three or more consecutive losses) often has a different profile, and aspirin doesn’t appear to address whatever is going wrong in those cases.

In the world of assisted reproduction, results are similarly mixed. One randomized trial in IVF patients reported impressive improvements in follicle count, oocyte retrieval, and pregnancy rates with aspirin.⁷ But other randomized, placebo-controlled trials in IVF found no significant effect on uterine artery blood flow.⁸ A trial of frozen embryo transfer cycles similarly found no statistically significant improvement in biochemical pregnancy, clinical pregnancy, or abortion rates in the aspirin group.⁹ The evidence in IVF is genuinely contradictory, and most reproductive endocrinologists will tell you it’s not a standard part of IVF protocols.

How Baby Aspirin Might Help Fertility

Aspirin inhibits an enzyme involved in producing prostaglandins, which are molecules that regulate inflammation, blood clotting, and blood vessel tone. The leading theory for how this helps with conception centers on blood flow to the uterus and ovaries. By reducing the production of thromboxane (a prostaglandin that constricts blood vessels), aspirin may promote better perfusion of the endometrial lining, making it more hospitable to an implanting embryo.

The clinical evidence for this mechanism is uneven. In women with thin endometrium undergoing insemination, aspirin significantly increased the rate of a “trilaminar” pattern (the three-layered appearance associated with receptivity), and pregnancy rates doubled compared with controls. But the same study found no significant increase in measurable endometrial thickness or blood flow resistance.¹⁰ Another trial in oocyte donation recipients with thin endometrium found no demonstrable increase in endometrial thickness at all.¹¹ Meanwhile, one IVF/ICSI trial found that while aspirin didn’t change average uterine artery blood flow, it did significantly reduce the incidence of “non-optimal” uterine blood flow patterns.¹²

So the picture is messy. Aspirin may not thicken a thin lining or dramatically improve average blood flow measurements, but it might shift the distribution so that fewer women end up with the worst blood flow patterns. Combined with its anti-inflammatory properties, which could help women with elevated CRP, the mechanism may be more about reducing interference with implantation than actively building a better lining.

Aspirin Alongside Ovulation-Inducing Medications

One area where aspirin has shown more consistent results is when used in combination with letrozole, a common ovulation-inducing drug prescribed for conditions like polycystic ovary syndrome (PCOS). A systematic review and meta-analysis found that adding aspirin to letrozole significantly increased endometrial thickness, the number of mature follicles, ovulation rates, and pregnancy rates while reducing miscarriage rates, compared with letrozole alone.¹³ The pregnancy rate roughly tripled in the combined-treatment groups across the pooled studies.

This makes physiological sense. Letrozole works by suppressing estrogen production temporarily, which stimulates follicle growth but can thin the endometrium as a side effect. Aspirin’s blood-flow-promoting properties may counteract that thinning effect. If your provider has you on letrozole for ovulation induction, the question of adding aspirin is worth raising.

Safety During TTC and Early Pregnancy

A dedicated safety analysis of the EAGeR trial looked at complications in women taking preconception aspirin. Rates of preterm contractions, subchorionic hemorrhage, epistaxis, kidney stones, placental abruption, and postpartum hemorrhage were similar between aspirin and placebo groups. The one difference: vaginal bleeding during pregnancy was more common with aspirin, affecting about 22% of the aspirin group versus 17% on placebo.¹⁴ That extra bleeding, while understandably alarming for anyone with a history of pregnancy loss, was not associated with increased pregnancy loss itself.¹

Some women worry about whether aspirin around the time of ovulation could interfere with the mechanics of ovulation itself, since other anti-inflammatory drugs like ibuprofen have been suspected of disrupting follicle rupture. A prospective cohort study tracking analgesic use around ovulation and implantation found that neither aspirin-type drugs, non-aspirin NSAIDs, nor acetaminophen were associated with reduced fecundability in any of the time windows examined.¹⁵ At the low doses used for fertility purposes (81 mg, compared with the 325 mg or more taken for pain), ovulation suppression does not appear to be a concern.

Bedtime Dosing May Matter

An underappreciated aspect of aspirin timing is what time of day you take it. A randomized trial studying aspirin’s effects on blood pressure and pregnancy complications found that the same 100 mg dose produced dramatically different outcomes depending on when it was swallowed. Aspirin taken upon waking had no effect on blood pressure compared with placebo. Aspirin taken eight hours after waking showed a significant blood-pressure-lowering effect, and aspirin taken at bedtime showed the greatest effect. Women who took aspirin in the morning or early afternoon had much higher rates of serious adverse pregnancy outcomes compared with those who took it in the evening or at bedtime.¹⁶

This trial was focused on preventing preeclampsia and related complications rather than on conception per se, but the physiological logic extends to TTC. Blood pressure regulation, vascular tone, and platelet activity all follow circadian rhythms. Taking aspirin at bedtime appears to align its peak activity with the body’s overnight vascular processes. While no trial has tested bedtime versus morning aspirin specifically for conception rates, the evidence is strong enough that many clinicians now recommend bedtime dosing as default.

The Preeclampsia Connection

If you start aspirin while TTC and become pregnant, you may already be covered for a separate indication: preeclampsia prevention. Guidelines from multiple professional bodies recommend low-dose aspirin for pregnant women at elevated risk of preeclampsia, with UK guidance specifying 75 to 150 mg daily starting from 12 weeks of gestation.¹⁷ Some evidence suggests that starting even earlier, before 11 weeks, provides better protection. If you have risk factors like a prior preeclamptic pregnancy, chronic hypertension, diabetes, or are carrying multiples, the aspirin you started for TTC purposes may transition seamlessly into preeclampsia prophylaxis. Discuss continuation timing with your provider once pregnant.

Interestingly, a recent randomized trial found that for women already on aspirin for preeclampsia prevention, discontinuing it between 24 and 28 weeks of gestation (if biomarker levels were reassuring) was not inferior to continuing through delivery.¹⁸ This means aspirin during pregnancy doesn’t have to be an all-or-nothing commitment through delivery, though that decision belongs firmly to your provider.

What About Your Partner’s Aspirin Use

Most conversations about baby aspirin and TTC focus entirely on the woman, but there is evidence that aspirin use by the male partner could cut the other way. A review of the scientific literature on aspirin and semen quality concluded that research consistently points to negative effects on sperm parameters, potentially through reduced testosterone synthesis, decreased prostaglandin formation in the testes, and increased oxidative damage to sperm.¹⁹ An in-vitro study confirmed that aspirin at physiologically relevant concentrations significantly reduced progressive sperm motility and vitality.²⁰ Animal research in rats showed that sustained aspirin exposure reduced sperm density and count in a dose- and time-dependent manner.²¹

Most of this research involved higher doses or longer exposure periods than the occasional headache pill, and the clinical significance for men taking low-dose aspirin for heart health isn’t settled. But if your male partner takes aspirin regularly and you’re having difficulty conceiving, it’s a factor worth mentioning to your fertility specialist. The irony of one partner taking aspirin to boost fertility while the other may be impairing it is not lost on researchers in the field.

Antiphospholipid Antibodies and Autoimmune Considerations

One group for whom preconception aspirin is less debatable is women with antiphospholipid antibodies. These antibodies increase the risk of blood clots in the placenta and are a recognized cause of recurrent miscarriage. Many clinicians recommend starting low-dose aspirin before conception in women with even low levels of these antibodies, sometimes adding heparin once pregnancy is confirmed if antibody levels are elevated.²² If you’ve been tested and found to have antiphospholipid antibodies, the timing question is simpler: start before conception and stay on it. This is one of the few fertility-related indications for aspirin where expert consensus is strong.

For women without a clear autoimmune diagnosis, the decision is more nuanced. The EAGeR trial results suggest meaningful benefit mainly for specific subgroups: women with a single prior loss, those with elevated inflammatory markers, and metabolically healthy individuals. If none of those descriptions fit, aspirin is unlikely to cause harm at the 81 mg dose, but the evidence that it will help is thin. A conversation with your provider about your individual history, blood work, and risk factors is the best way to decide whether preconception aspirin belongs in your TTC plan.