The following information may help to address your question:
According to Rodríguez (2007) et al., all the cases of trisomy 18p show a noncharacteristic facial appearance and a mental development ranging from normal to moderate impairment.
Examples of Cases:
Some have suggested that several of the cases of trisomy 18p reported in the medical literature of people with signs and symptoms represent cases in which other chromosomal abnormalities not yet identified may explain the signs and symptoms present, suggesting that pure trisomy 18p results in little, if any, signs and symptoms. As more advanced techniques for identifying chromosomal abnormalities become available, we will learn more about the signs and symptoms associated with trisomy 18p. It is possible that some cases of trisomy 18p are being missed because they do not have any signs and symptoms and are of normal intelligence.