An evaluation of overlying tissues to determine fetal exposure to ultrasound during the third trimester.

Measurements of minimum and average total tissue thicknesses overlying the fetus during the third trimester were taken on 52 patients between 24 and 40 weeks gestation. Minimum total thickness of tissues constantly along the ultrasound path had a mean value of 1.75 cm, ranging from 0.40 to 3.70 cm. This corresponded to a mean attenuation of 0.87 dB MHz-1, ranging from 0.30 to 1.68 dB MHz-1. Average total thickness of tissues constantly along the ultrasound path had a mean value of 2.16 cm, ranging from 1.00 to 4.10 cm. This corresponded to a mean attenuation of 1.16 dB MHz-1, ranging from 0.64 to 2.03 dB MHz-1. Average attenuation was also calculated for all soft tissues along the ultrasound path, including the placenta. The resulting average attenuation was 0.47 dB cm-1 MHz-1. Maternal weight at the time of ultrasound examination had significant correlation with minimum and average ultrasound attenuation by tissues constantly along the ultrasound path. The R2 value was 46.7% for minimum attenuation (p-value < 0.0001) and 53.3% for average attenuation (p-value < 0.0001). Multiple regression analyses showed that the model that included maternal weight, pregnancy weight and fetal gestational age was a better predictor of the amount of attenuation during the third trimester than the model that included maternal weight only. The R2 value of the model that included all three variables was 57.2% (p-value < 0.0001) for minimum attenuation, and 62.4% (p-value < 0.0001) for average attenuation.