How do doctors get the sex wrong on a sonogram / ultrasound?

Best estimate: It is not so much that it is wrong, but more that it is the best estimate at the time with the information given. Ultrasounds are not high resolution pictures but more a grainy picture of a moving object on a small screen. In addition, ther can be anatomical similarities between male and female genitalia on an ultrasound. Remember that nothing is 100% when it comes to test results.

Not always easy 2 do: Best images of a fetus are obtained by being able to get multiple views of the baby 'parts'/in right position/through thin layers of the abdominal wall of the expectant woman. It is best to not get the family excited and expect that gender will be determined on an early ultrasound. If 'boy parts' are seen in the second or third trimester, then it probably is a boy. What does this photo tell y'all?

Embryologic similar: The male and femal genitalia form from the same embryonic precursor and can look similar early in the gestation. For instance, the penis and clitoris are formed from the same precursor, as are the scrotum and labia.

It is a prediction: A prenatal ultrasound scan can predict fairly accurately a baby's gender, but the prediction is not 100% accurate. One should not think of an incorrect prediction as a mistake, because a prediction is just a prediction. Unless parents have a genetic problem that either affects only boys or only girls, not knowing the gender until birth is very exciting! ultrasounds were not even used until 1960's!