Tsarna E, Reedijk M, Birks LE, et al Maternal Cell Phone Use During Pregnancy, Pregnancy Duration And Fetal Growth In Four Birth Cohorts. Am J Epidemiol. 2019 Apr 17. pii: kwz092. doi: 10.1093/aje/kwz092.
Previous studies evaluating potential effects of prenatal exposure to radiofrequency fields from cell phones on birth outcomes are inconsistent. We explored if maternal cell phone use was associated with pregnancy duration and fetal growth. We used information from 55,507 pregnant women and their children from Denmark (1996-2002), the Netherlands (2003-2004), Spain (2003-2008) and Korea (2006-2011). Based on self-reported number of cell phone calls per day, exposure was grouped as none, low (reference level), intermediate, and high. We examined pregnancy duration (gestational age at birth, preterm/postterm birth), fetal growth (birth weight ratio, small/large for gestational age), and birth weight, low and high birth weight, and meta-analyzed cohort specific estimates. The intermediate exposure group had higher risk of giving birth at lower gestational age (Hazard Ratio=1.04, 95%CI 1.01, 1.07), and exposure-response relationships were found for shorter pregnancy duration (P<0.001) and preterm birth (P=0.003). We observed no association with fetal growth or birth weight. In conclusion, maternal cell phone use during pregnancy may be associated with shorter pregnancy duration and increased risk for preterm birth. Results should be interpreted with caution, as they may reflect stress during pregnancy or other residual confounding, rather than direct effect of cell phone exposure.
Lennart Hardell's blog 
Caution, yes, but EMF causes stress anyway.