Environmental factors during pregnancy play a critical role in shaping fetal development, with potential lifelong health effects. In particular, imbalances in metallic element exposure have been associated with adverse pregnancy outcomes such as fetal growth restriction (FGR). Rubidium (Rb) is of particular interest due to its widespread presence and possible impact on prenatal growth. Although reports are limited, animal studies suggest that Rb may play a role in fetal development. For instance, one study showed reduced prenatal growth with low dietary Rb in goats. These findings are intriguing but lack consideration of essential growth determinants, such as maternal demographic and obstetric data, and have not yet been extended to human populations.

Here, we present the results of a pilot study that was conducted involving 99 low-risk human pregnancies with amniotic fluid samples collected at 19 weeks gestation. The Rb concentrations in the amniotic fluid and their correlation with fetal growth, including birthweight and weight trajectories. Maternal, fetal, and newborn characteristics were analyzed. Lower Rb concentrations were found to be significantly associated with small for gestational age newborns (106 vs. 136 μg/L, p < 0.01) and reduced fetal weight growth trajectories. A Rb threshold of 122 μg/L was identified, with fetuses below this level exhibiting a three-fold higher incidence of decreased fetal weight percentiles (OR=3.2, P=0.03). The daily percentile ratio index was negative for rubidium concentrations below 122 μg/L, indicating reduced growth trajectories (P=0.02).

These findings suggest that Rb concentration in mid-pregnancy amniotic fluid could serve as a potential predictor for fetal growth patterns and birthweight classification, particularly in predicting lower birth weights and decreased fetal growth. Further investigation is required to understand the role of Rb in fetal development and its potential as a diagnostic marker.