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Preterm Birth Rates Higher in Third Trimester for SARS-CoV-2-Infected Mothers

Preterm Birth Rates Higher in Third Trimester for SARS-CoV-2-Infected Mothers

Preterm Birth Rates Higher in Third Trimester for SARS-CoV-2-Infected Mothers

Preterm Birth Rates Higher in Third Trimester for SARS-CoV-2-Infected Mothers

Credit: Camylla Battani on Unsplash.
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A new research study of over 5,000 pregnant women has found that infection with SARS-CoV-2 is associated with increased risk of preterm birth. However, this increased risk only exists if women are infected in their third trimester. The results are published in PLOS ONE.

Infectious diseases and pregnancy

Since the onset of the COVID-19 global pandemic, scientists have continued to explore the different ways that SARS-CoV-2 can impact the human body. This includes studying otherwise healthy individuals that become infected, those that may be classed vulnerable due to existing medical conditions and pregnant women.

Navigating a global pandemic is challenging in itself, without the additional consideration of having to protect yourself and your baby from infection with SARS-CoV-2. While existing research suggests that pregnant women are at a higher risk of developing serious illness from COVID-19, data on the effects that SARS-CoV-2 infection at different stages of pregnancy can have on birth outcomes is limited.

We know that other infectious diseases can have varying impacts on pregnancy outcomes. A 2020 study published in Lancet Infectious Diseases identified an association between influenza infection during pregnancy and reduced birth-weight, in addition to an increased risk of late pregnancy loss. Participants with respiratory illness and fever were also found to be at increased risk of having a preterm birth.

In 2002–2004, a different member of the coronavirus family – severe acute respiratory syndrome coronavirus (SARS-CoV) – caused an epidemic, resulting in an estimated 774 deaths worldwide. While documented cases of SARS-CoV infection during pregnancy are scarce, small clinical studies describe an association with maternal death and spontaneous abortion.

Understanding exactly how COVID-19 infection – occurring at different stages of pregnancy – impacts maternal and fetal outcomes is therefore paramount to ensure that mothers can best protect their own and their babies’ health.

Noga Fallach, a biostatistician and epidemiologist at the Kahn-Sagol-Maccabi Research and Innovation Center and colleagues conducted a new study exploring preterm birth (PTB) and small-for-gestational-age (SGA) rates in pregnant individuals infected with SARS-CoV-2 at different stages of pregnancy.

How were the study outcomes defined?

Preterm birth – defined as less than 37 weeks of gestation.

Small-for-gestational-age – defined as infant body weight at birth less than the gender-specific percentile for the gestational age.

Fallach and colleagues utilized anonymized data captured by the Maccabi Healthcare Services in Israel from February 21 2020 to July 2 2021, and matched 2,753 pregnant women infected with SARS-CoV-2 during pregnancy with 2,753 pregnant women that were not infected. The prevalence of other health conditions, such as cancer, diabetes, hypertension and cardiovascular diseases – which could be considered confounding factors – was described as “low and similar in both groups”, by the researchers.

Higher rate of PTB in third trimester for women infected with SARS-CoV-2

From the SARS-CoV-2 infected cohort, 17.4% were infected during the first trimester, 34.2% in the second and 48.4% in third trimester. Statistical analyses revealed that women infected with SARS-CoV-2 during the first and second trimesters of pregnancy were not associated with an increased risk of PTB. Those infected with SARS-CoV-2 in their third trimester, however, were 2.76 times more at risk of PTB. Specifically, women infected >34 weeks gestation were 7 times more likely to experience PTB. When trimester was not accounted for, rate of PTB was not found to be significantly different across both infected and non-infected groups. The researchers say that it can be assumed clinicians chose to induce women with symptomatic COVID-19 due to their condition.

Focusing on SGA, the research team did not identify any difference between groups, regardless of trimester. Pregnancy loss rates were also similar in those infected with SARS-CoV-2 (3.9%) and non-infected women (3.8%).

“The results are encouraging and reassuring that COVID-19 infection during pregnancy is not associated with any type of pregnancy loss,” said Dr. Tal Patalon, head of KSM and a co-author of the study. Based on their findings, the research team advise that women in their third trimester should take particular care to reduce their risk of becoming infected with the virus.

The timeline of the study also means that the research does not account for Delta variants, which emerged after the study closed. Insights on the effects of the Omicron variant, which is the most dominant variant found today, in pregnant women cannot be drawn from this study.

The research team plan to further investigate other short- and long-term outcomes of infants born to infected mothers.

Reference: Fallach N, Segal Y, Agassy J, et al. Pregnancy outcomes after SARS-CoV-2 infection by trimester: A large, population-based cohort study. PLOS ONE. 2022. doi: 10.1371/journal.pone.0270893.

Meet the Author
Molly Campbell
Molly Campbell
Senior Science Writer