The info she has been given doesn't make sense.

Every person has 2 chomosome 18s. You get two copies of every chromosome. Having 3 copies of a chromosome or only one copy can cause issues, but this is usually diagnosed via CVS or amnio. There can also be a segment of the chromosome missing or extra. They can do amnio up to just before birth.

She needs to ask for a second opinion, or get a genetic counsellor or social worker (in a smaller hospital) to find out and explain what is happening.