If form holds, the 250,000 women each year who undergo the same genetic testing Jolie had will be joined by the thousands more; but the mutation that was detected in her, in what's known as BRCA1 gene, is ~~.
"If the trend continues":
manner, method, or style of doing something, esp with regard to recognized standards
The most common idiom in US is "true to form" idioms.thefreedictionary.com/true+to+form Jun 17, 2013 at 12:37
Yeah, that was what I went for. I have never heard of "if form holds" However it is from a senior editor at Time mag: healthland.time.com/author/jkluger - seems to be more of a sports expression Jun 17, 2013 at 13:17
"If the pattern holds..." Although when I read it out of context I thought it meant: "If propriety holds..."