Mediterranean Diet in Early Pregnancy and Preterm Birth

Starting a Mediterranean-style diet in the first trimester may lower the risk of preterm birth, but the headline number needs an asterisk large enough to see from the produce aisle. A 2023 meta-analysis reported a pooled relative risk of 0.45 (95% CI, 0.26–0.76) across seven randomized trials: a 55% lower relative risk among intervention participants. The same analysis found that trials beginning in the first trimester had stronger results than those beginning later in pregnancy (subgroup heterogeneity P=0.02).

That is a clinically interesting finding. It is not proof that a Mediterranean diet will prevent preterm birth for every pregnant person, or even that the estimated 55% reduction will hold up in another large review. Most participants in the wider evidence base were White European women, while observational studies in more diverse, lower-income U.S. populations have produced inconsistent results.

The result that drew attention

In a 2023 systematic review and meta-analysis by Yang et al., researchers examined Mediterranean diet and female reproductive health across 32 studies involving 103,204 participants. The preterm-birth estimate came from seven randomized controlled trials within that review: relative risk 0.45, with a 95% confidence interval from 0.26 to 0.76. Because the interval stays below 1.0, the pooled result was statistically significant.

Relative risk is useful, but absolute risk decides how much the finding changes a real pregnancy. If a group has a 10% baseline preterm-birth rate, a relative risk of 0.45 would correspond to an estimated absolute reduction of about 5.5 percentage points—from 10% to 4.5%—and a number needed to treat of about 18. That calculation is an illustration based on the pooled estimate and a 10% baseline rate, not a result directly measured in the trials.

The confidence interval matters as much as the point estimate. It allows effects ranging from a 24% relative reduction to a 74% relative reduction. The analysis also did not report the I² heterogeneity statistic for this endpoint, and its published summary does not break out the seven individual trial effects, sample sizes, or control conditions. Those gaps make it hard to tell whether several similar trials produced a stable result or whether a small number of unusually positive trials drove the average.

Why timing is plausible—and still unproven

The first-trimester subgroup result gives the finding biological logic. Early pregnancy includes trophoblast invasion and spiral-artery remodeling, processes that establish placental blood flow. A diet built around vegetables, fruit, legumes, whole grains, olive oil, fish, and nuts could plausibly affect inflammatory or metabolic conditions during that period.

But plausibility is not a causal pathway. The Mediterranean diet contains fiber, omega-3 fats, and polyphenol-rich foods, and inflammation plays a role in some pathways to preterm labor. Yet the available randomized evidence did not show that this dietary pattern changes prostaglandins, IL-6, TNF-α, or CRP in a way that explains fewer preterm births.

A 2022 PACE Consortium meta-analysis by Küpers et al. adds a reason to take pregnancy timing seriously. Higher maternal Mediterranean-diet adherence was associated with differences in newborn DNA methylation at CpG sites linked to cardiometabolic programming. This shows an association between maternal diet and a newborn biomarker, not that methylation changes caused lower preterm-birth risk. It also does not establish that first-trimester dietary changes produce effects that later changes cannot.

The timing signal itself came from a subgroup analysis within the Yang meta-analysis, rather than a prespecified primary outcome in a dedicated randomized trial. A P value of 0.02 is worth investigating, but subgroup findings can emerge by chance when researchers compare several categories. The practical reading is simple: starting early is sensible if someone wants to adopt the pattern, but the evidence has not yet identified an exact intervention window with certainty.

The broader pregnancy evidence is encouraging, not identical

Mediterranean-pattern eating has shown a broader favorable signal across pregnancy outcomes. Yang et al. reported lower pooled risks of gestational diabetes (RR 0.74, 95% CI 0.55–0.99) and gestational hypertension (RR 0.71, 95% CI 0.51–1.00). Its estimate for preeclampsia was RR 0.82 (95% CI 0.54–1.22), which includes the null and therefore does not establish a statistically significant reduction in that analysis.

Two newer reviews support the general direction without independently confirming the preterm-birth number. A 2024 systematic review and meta-analysis by Xu et al. found beneficial associations between maternal Mediterranean diet and perinatal outcomes, while a 2025 systematic review by Sharma et al. reported lower risks of gestational diabetes, preeclampsia, and excessive gestational weight gain among people with higher adherence. Neither review supplied a new pooled relative-risk estimate for preterm birth that could verify the 0.45 figure.

Randomized evidence exists for other pregnancy endpoints. In the IMPACT BCN randomized clinical trial, 1,221 pregnant individuals at elevated risk received a Mediterranean diet intervention or mindfulness-based stress reduction to prevent small-for-gestational-age birth weight. The Mediterranean diet intervention reduced small-for-gestational-age births in that enrolled high-risk population. The trial was designed around small-for-gestational-age birth weight, not as a definitive preterm-birth trial, so it should not be used as an independent confirmation of the meta-analysis's preterm-birth effect.

Who was studied changes what the number means

More than 95% of participants across the 32 studies in Yang et al.'s review were White European women. That is the largest limitation of the preterm-birth claim. Preterm birth carries a disproportionate burden among Black, Hispanic, and South Asian populations in many high-income settings, yet those groups are underrepresented in the trials that produced the pooled estimate.

U.S. observational research complicates the picture. A 2022 cohort study by Makarem et al. found inverse associations between Mediterranean diet adherence and adverse pregnancy outcomes among U.S. women, offering some support outside the European trial base. Association does not settle causation, though; people who eat this way may also differ in healthcare access, income, education, stress, or other factors that affect pregnancy outcomes.

More pointedly, a 2021 study by Rhee et al. found inconsistent associations between Mediterranean-style diet adherence and birth outcomes in an urban, multiethnic, low-income U.S. population. A separate early-pregnancy cohort study by Yisahak et al. also reported dietary-pattern relationships with neonatal outcomes that did not consistently match the large protective effects suggested by the pooled trials.

These studies do not overturn randomized trials conducted elsewhere. They do show why a single effect estimate should not travel farther than its evidence does. Differences in food access, baseline diet, structural inequities, medical risk, and how investigators measured adherence could all change the observed relationship.

What to do with this now

A Mediterranean-style pattern is a reasonable low-risk choice for people planning pregnancy or entering early pregnancy: emphasize vegetables, fruit, beans, whole grains, nuts, olive oil, fish, and other minimally processed foods. It may support several pregnancy outcomes, and the preterm-birth signal is substantial enough to justify better trials.

It should not replace established obstetric care or prevention strategies for people with known preterm-birth risk. No trial in this evidence base compares Mediterranean diet counseling head-to-head with vaginal progesterone, cervical cerclage, or omega-3 supplementation. The estimated number needed to treat of 18 also cannot rank the diet against those interventions; in high-risk women, vaginal progesterone has reported numbers needed to treat around 7 to 10.

The current evidence supports an early Mediterranean-style diet as a sensible pregnancy habit with a plausible and potentially large preterm-birth benefit in the populations studied. It does not yet support promising every pregnant person a 55% reduction in risk.

Early nutrition can matter without making every early-nutrition claim true. The NiPPeR randomized trial tested a preconception supplement containing myo-inositol, probiotics, and micronutrients against a standard micronutrient supplement in 1,729 women planning conception; it did not produce the anticipated reduction in gestational complications. Timing is a serious hypothesis, not a substitute for proof.