In this issue of JAMA Pediatrics, Martens et al1 report an inverse association between exposure to air pollution during the second trimester of pregnancy and relative telomere length (RTL) in cord blood samples and placentae. Monitoring stations calibrated to estimate exposure to particles of diameter less than or equal to 2.5 µm (PM2.5) measured exposure levels at the maternal residence. The RTL was measured using quantitative polymerase chain reaction (qPCR). If confirmed, this intriguing finding could help elucidate the association of environmental exposures with telomere length (TL) dynamics in utero, the period of most intense cell division during the entire human life course. In this editorial, we briefly discuss 4 topics relevant to interpretation of the key finding and related results: (1) biological meaning, (2) TL measurement, (3) timing of prenatal exposure, and (4) approaches to further testing of validity.