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The prevalence of autism spectrum disorder (ASD) has been increasing rapidly, with current estimates of 1 in 68 children affected. Simultaneously, use of prenatal ultrasonography has increased substantially, with limited investigation into its safety and effects on brain development. Animal studies have demonstrated that prenatal ultrasonography can adversely affect neuronal migration.To quantify prenatal ultrasound exposure by the frequency, timing, duration, and strength of ultrasonographic scans in children with later ASD, developmental delay, and typical development.This case-control study included 107 patients with ASD, 104 control individuals with developmental delay, and 209 controls with typical development. Participants were identified from medical records based on prenatal care and delivery at Boston Medical Center, a diverse, academic, safety-net medical center, from July 1, 2006, through December 31, 2014, with a gestational age at birth of at least 37 weeks. Data were analyzed from May 1, 2015, through November 30, 2017.Ultrasonographic exposure was quantified by the number and timing of scans, duration of exposure, mean strength (depth, frame rate, mechanical index, and thermal index), and time of Doppler and 3- and 4-dimensional imaging.Among participants with ASD and controls with developmental delay and typical development, ultrasound exposure was quantified and compared per trimester and for the entire pregnancy, with adjustment for infant sex, gestational age at birth, and maternal age.A total of 420 participants were included in the study (328 boys [78.1%] and 92 girls [21.9%]; mean age as of January 1, 2016, 6.6 years; 95% CI, 6.5-6.8 years). The ASD group received a mean of 5.9 scans (95% CI, 5.2-6.6), which was not significantly different from the 6.1 scans (95% CI, 5.4-6.8) in the developmental delay group or the 6.3 scans (95% CI, 5.8-6.8) in the typical development group. Compared with the typical development group, the ASD group had shorter duration of ultrasound exposure during the first (290.4 seconds [95% CI, 212.8-368.0 seconds] vs 406.4 seconds [95% CI, 349.5-463.3 seconds]) and second (1687.6 seconds [95% CI, 1493.8-1881.4 seconds] vs 2011.0 seconds [95% CI, 1868.9-2153.1 seconds]) trimesters but no difference in the number of scans. The ASD group had greater mean depth of ultrasonographic penetration than the developmental delay group in the first trimester (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.1-12.1 cm]). The ASD group had greater mean depth than the typical development group during the first (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.3-12.0 cm]) and the second (12.9 cm [95% CI, 12.6-13.3 cm] vs 12.5 cm [95% CI, 12.2-12.7 cm]) trimesters.This study found significantly greater mean depth of ultrasonographic penetration in the ASD group compared with the developmental delay group in the first trimester and compared with the typical development group in the first and second trimesters. Further research is needed to determine whether other variables of ultrasound exposure also have adverse effects on the developing fetus.