By contrast, the scientists say their new test would identify far more conditions, caused by genetic errors.
However, they warned it raised many ethical questions because the results could be used as a basis for abortion.
These concerns were last night amplified by pro-life campaigners, who said widespread use of such a test would inevitably lead to more abortions.
The American scientists were able to map the babys genetic code principally from tiny traces free-floating DNA, which makes its way into the mothers blood.
Blood sample DNA from the mother was also studied as well as DNA extracted from the father's saliva.
Fitting pieces of the genetic jigsaw together, scientists in the US were able to reconstruct the entire genetic code of an unborn baby boy.
They were then able to see what spontaneous genetic mutations had arisen.
Such natural mutations - called de novo mutations - are responsible for the majority of genetic defects.
By checking their prediction of the babys genetic code with actual DNA taken after the birth, the team from the University of Washington in Seattle, found they were able to identify 39 of 44 such mutations in the child.
De novo mutations are thought to play a role in a number of complex conditions such as autism and schizophrenia.
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Unborn babies could be tested for 3,500 genetic faults