There is a real and substantial risk to pregnant women in those scanners. Babies undergo very fast (stressed) cellular replication. Small alterations in bio-chemical processes, DNA mutagenesis or anything minor like this can cascade down the line much faster than in an adult and have huge long-term effects on a baby.

For the TSA to be telling people the machines are "safe" for children and pregnant women is utter insanity and it tells you exactly how much you can't trust ANYTHING they say because what they are saying is directly conflicted with the laws of physics and biochemistry.