Being told once is not the same as understanding
False beliefs about treatment survive being explained. That is a finding about how people work, not a failure of character.
There is a well-established gap between having been told something and having taken it in. Research ethics has studied it for forty years, because consent forms depend on it.
The classic description, from 1987: Using examples from psychiatric research, the authors explore the ethical dilemma of the 'therapeutic misconception,' where, despite explanation, patient-subjects believe that research protocols are designed to benefit them directly rather than to test or compare treatment methods. Even potential subjects who demonstrate an understanding of randomization, double-blinded studies, and the use of placebos frequently persist in a belief that they purposely will receive the treatment most likely to benefit them.
The two words that matter are despite explanation. People who could correctly describe how the study worked still believed it was designed for them personally. Explanation did not dislodge it.
A second and separate idea, often confused with the first, is unrealistic optimism: believing that your own result will be better than that of other people in exactly your position. A review that keeps the two apart makes the point that they are different problems needing different remedies.
Now think about what a pregnant woman is told. Eat this. Walk that far. Do your exercises. Sleep when the baby sleeps. Every one of those raises the sense that the result is up to her. On this evidence, that is exactly the input that produces unrealistic optimism, and when things then go badly it produces self-blame rather than a phone call to the clinic.
This is reasoning by analogy from cancer-trial patients to pregnant women. It has not been demonstrated in pregnancy. Treat it as a reason for care in how advice is phrased, not as a proven result.
"Is there anything I am assuming that is not true?"
"If this goes wrong, will it be because of something I did?"