Some women may be deciding to have abortions on the back of this poor advice. The researchers say their findings prove the need for protocols to standardise the way potentially disturbing information is given to parents.
Most people have two chromosomes which determine their sex - girls have XX and boys have XY.
However, some girls have an extra X and some boys have an extra X or Y. These additional chromosomes may create some health problems, but they are usually not severe.
For instance, females with triple X syndrome may have slight learning difficulties, be emotionally immature and taller than average, but will have normal sexual development and will be fertile.
Enormous variation
Lenore Abramsky and colleagues from Imperial College of Science, Technology, and Medicine, in London, UK, interviewed 29 health professionals who had recently informed parents that their apparently normal foetus was carrying an extra sex chromosome.
Some obstetric units had no established protocol for communicating results to parents.
Although there were some examples of excellent counselling, there were other examples of grossly inadequate or frankly misleading information being given, say the authors.
One woman was told her unborn son had the condition Klinefelter's syndrome. Babies with this condition are infertile and may have a slightly lower IQ.
However, the woman had an abortion without even seeing a genetics expert after a doctor told them their child would be mentally retarded.
Good practice
In the British Medical Journal, the authors argue that it is essential for obstetric units to have an established protocol for giving results to parents.
They also stress the need for all staff who communicate with parents to have accurate, up-to-date information about the condition identified.
Ms Abramsky told BBC News Online that there was no evidence that all parents were given the correct diagnosis, and that the information they received about their child's condition was first rate when they got to see a specialist in genetic medicine.
She said: "It is disturbing to note the haphazard nature of how parents were informed of the diagnosis, what information was given, and what was implied.
"We should not be increasing people's anxiety. We want to make sure that somebody is in a frame of mind to listen when they are talking to an expert, and not so panicked that they cannot take things in."