Women who received their first mRNA COVID-19 shot during early pregnancy, specifically between weeks eight and 13, were significantly more likely to experience pregnancy loss than expected, according to a recent study in Israel. The study, which is the first national-scale analysis to examine gestational timing in relation to receipt of the mRNA COVID biologic during pregnancy, found nearly four additional losses per 100 pregnancies in this group. This represented a roughly 40 percent increase over the baseline rate.1
Most of the pregnancy losses occurred during late-term pregnancy, with the majority happening after 20 weeks gestation and nearly half after 25 weeks. This type of pregnancy loss is categorized as a stillbirth, which is associated with greater medical risks and profound psychological impacts for mothers.1
The large retrospective cohort study, published in June 2025 as a preprint on medRxiv, analyzed data from 226,395 recorded singleton pregnancies (pregnancies with only one fetus) between 2016 and 2022 through Leumit Health Services, one of Israel’s largest health maintenance organizations. The increased risk was identified specifically during weeks eight to 13 of gestation, a critical period for fetal development and placental formation.
Women who received an mRNA COVID shot prior to pregnancy or later in gestation did not experience higher-than-expected fetal losses. This suggests a potential window of biological vulnerability early in pregnancy that has not been thoroughly studied or acknowledged by public health authorities.1
Flu Shot During Pregnancy Appears Safer, But Questions Remain
In contrast to the elevated rate of fetal loss observed following first-trimester mRNA COVID vaccination, the study found that women who received influenza vaccine during pregnancy experienced fewer fetal losses than expected. These findings suggest that the increased rate of fetal loss may be specific to the mRNA platform or ingredients in the COVID biologic specifically.1
However, the authors of the study caution that other factors could have influenced the results. “This might suggest a healthy vaccinee bias where individuals who choose to be vaccinated are generally healthier and more engaged with healthcare services,” the wrote. In other words, women who opt for a flu shot during pregnancy may already be more likely to have better prenatal care or fewer underlying risk factors, which could lower their likelihood of complications regardless of vaccination.1
Notably, the timing of influenza vaccination often occurs later in pregnancy, potentially avoiding the more vulnerable developmental window identified in the COVID shot group. The researchers also noted that “some of the association we observed may be attributable to residual confounding,” meaning there may be other unmeasured variables influencing the results.1
Czech Study Found Lower Conception Rates Among Women Who Got mRNA COVID Shots
The Israeli study came out just a day after a large-scale population study conducted in the Czech Republic found that women who received an mRNA COVID shot prior to conception were significantly less likely to achieve a successful pregnancy compared to their peers who did not get the shot.2
The study, published in the International Journal of Risk & Safety in Medicine, used nationwide health data from the Czech Republic between 2021 and 2023 and tracked conception rates—defined as pregnancies that resulted in live births—among women aged 18 to 39 who did and did not receive the mRNA biologic prior to conception. From mid-2021 onward, monthly conception rates among women who received an mRNA COVID shot were up to 1.5 times lower than those of women who did not get the injection, despite women who got the mRNA shots comprising roughly 70 percent of the reproductive-age population.1
Both studies highlight timing-specific associations that have not been widely explored in earlier clinical research. The Israeli data focuses on pregnancy losses following early gestational vaccination, while the Czech study tracks reduced conception success following preconception vaccination. These studies emerge amid a continued global decline in birth rates and increased scrutiny of recommendations by public health officials.1 2
Unlike most pregnancy loss studies, which primarily assess early first-trimester outcomes, the Israeli study identified a disproportionate number of fetal losses occurring after 20 weeks gestation, which indicates the need for further investigation of whether mRNA COVID shot exposure during early pregnancy could influence later-stage fetal development or placental function. Together, the Israeli and Czech findings contribute to a growing body of observational data suggesting that the effects of mRNA COVID vaccination may extend beyond short-term menstrual changes and could include potential impacts on conception, implantation, or gestational development, particularly when exposure occurs during specific developmental window.1 2
CDC Revises COVID Shot Recommendations for Children and Pregnant Women
Following the call by Health and Human Services (HHS) Secretary Robert F. Kennedy, Jr. for a federal ban on administering COVID shots during pregnancy, citing a lack of long-term fertility safety data, the U.S. Centers for Disease Control and Prevention (CDC) revised its vaccination guidance for children and pregnant women.3
As of June 2025, the CDC no longer recommends that these groups should be given COVID shots.. Instead, the federal health agency now advises that individuals in these categories may choose to receive the vaccine in consultation with their healthcare providers—a shift toward “shared decision-making” which public health experts hope will increase vaccine uptake.3
“The old COVID-19 vaccine recommendations for healthy children under 18 and for pregnant women have been removed from the CDC vaccine schedule,” an HHS spokesperson said in a prepared statement. “The CDC and HHS encourage individuals to talk with their healthcare provider about any personal medical decision.”3
