The results differ from previous studies and the predictions of the researchers that the Child Tax Credit would reduce the prevalence of low birth weight among pregnant women across the country.
The odds of low birth weight (LBW) among birth cohorts exposed to the Child Tax Credit (CTC) increased above expected values in 5 of the 6 months of the CTC advance payments, according to the results of a study published in the Journal of the American Medical Association.
Infants and pregnant people in the United States fare worse on measures of preterm delivery, infant mortality, maternal mortality, and LBW. Additionally, US families are more likely to live in poverty with a less generous social safety net, which has led to discussions regarding whether economic conditions contribute to this disparity.
Income transfer programs, such as the CTC, can provide money to low-income families and are designed to support increased consumption and asset building. More than 55% of American adults with children reported receiving CTC advance payments, which reduced the number of children living in poverty by 34% and reduced household food insufficiency by 26%, according to the study.
Because changes in maternal nutrition can affect fetal growth, the investigators hypothesized that the documented improvement in nutrition due to CTC payments may have increased fetal growth among those eligible for the CTC. The final sample size was identified using data from the National Center for Health Statistics Vital Statistics birth certificate records from January 2014 through December 2021, and totaled 28,866,466.
Parous people contributed to 61.2% of those births during the study period. The crude prevalence of LBW in the analytic population was 6.5%, and among parous people, the prevalence was 5.8% (n = 1,024,153).
Next, investigators identified and estimated a Box-Jenkins transfer function for the log odds of LBW among the 90 pre-CTC monthly birth cohorts. The log odds of LBW among parous people and among the comparison population shared a rising trend. Regressing the former on the latter, and on preterm birth among parous people, produced residual LBW values that remained stationary in their mean.
The researchers then applied the previously used transfer function to all 96 cohorts. The observed LBW values for the last 5 cohorts appeared greater than expected (range of increases, 3.3%-5.4% across the 5 months).
Finally, the investigators used an outlier-adjusted regression, which yielded a coefficient for the CTC binary variable of 0.042 (SE, 0.008), which implies that the odds of LBW increased an average of 4.2% (95% CI, 2.7%-5.7%) among the cohorts born to parous people in the past 5 months of 2021.
These findings were contrary to the hypothesis put forth by the investigators and differ from previous studies in the United States and Canada that showed positive associations of income transfers with birth weight and reductions in LBW. However, differing from prior studies, this current study accounted for time-dependent autocorrelation in the outcome in the test group, identified a clear comparison group, or targeted the exact timing or receipt of income transfers.
The investigators discussed multiple post hoc explanations for their findings. First, families may have used the CTC payments to benefit existing children and not the pregnant person, which would suggest a null result. Second, it has been reported that there was a statistically detectable spike in fertility following pandemic shutdowns, which could have temporarily lowered the rate of LBW if it reflected a transient shift in the composition of the reproducing population.
Third, some prior literature has suggested that cash transfers such as the CTC are associated with increased risky health behaviors, though the researchers noted they were unaware of evidence in pregnant populations. Despite this, if even a small number of pregnant people—or people in their households—used the CTC in ways that negatively influenced health, it could result in an increase in LBW.
The final explanation arises from the theory of selection in utero, which states that natural selection-conserved mechanisms spontaneously abort smaller fetuses, particularly in suboptimal or threatening environments. Investigators postulated that the CTC benefit could have decreased the standard for how large a fetus must be to avoid spontaneous abortion, implying that fetuses that would otherwise have been spontaneously aborted instead survived until birth, but with increased odds of LBW.
“We encourage replication and extension of our work to examine the outcomes of the CTC expansion on parental and child health domains over the life course, which may be complex and varied based on prior examinations of income transfers on health,” the investigators concluded.
Margerison CE, Zamani-Hank Y, Catalano R, et al. Association of the 2021 Child Tax Credit advance payments with low birth weight in the US. JAMA Netw Open. 2023;6(8):e2327493. doi:10.1001/jamanetworkopen.2023.27493