The ruling is a blow to Sequenom, a California company that introduced the first such noninvasive test in 2011 and has been trying to lock out competitors in a fast-growing market by claiming they infringe on the patent.
Sequenom’s stock fell 23 percent on Thursday, to $1.92.
The judge, Susan Illston of the United States District Court in Northern California, issued a ruling on Wednesday that the patent was invalid because it covered a natural phenomenon — the presence of DNA from the fetus in the mother’s blood.
The ruling was a sign that the Supreme Court’s decision in June declaring that human genes may not be patented because they are products of nature could make it more difficult to patent diagnostic techniques.
Judge Illston cited the gene patent case, which involved Myriad Genetics, in her ruling, along with a 2012 Supreme Court decision invalidating patents on a test used to determine the proper...