- The data available to data suggest that diagnostic US has no adverse effect on embryogenesis or fetal growth. However, although B and M mode are safe during the first trimester, color, pulsed or power Doppler should be performed with caution. The US effects are mainly due to cavitation. However, this Mechanism has been determined mainly in animal models. Thermal effect, which was thought to be hazardous, probably does not influence fetal development.