Ronan Farrow explains the Harvey Weinstein trial

The prosecutors in Harvey Weinstein's rape and sexual assault trial in New York City are taking a "big risk" in expanding his case, according to journalist Ronan Farrow.

Weinstein, 67, is facing five felony charges, including predatory sexual assault, and he could go to prison for life if convicted on all counts. Six women are expected to testify during the trial that they were sexually assaulted by the Hollywood movie producer, though he is only charged in this case with assaulting two of them.

The other women are expected to testify because prosecutors are seeking to show Weinstein had a pattern of predatory behavior. It's that expansion that Farrow, who reported allegations against Weinstein for The New Yorker, told "Good Morning America" Tuesday is a risk as they'll "have to sort of double prove" charges.

"They have to prove not only the charge at issue but also this secondary witness they're bringing in," Farrow told ABC News Chief Anchor George Stephanopoulos.

"It is difficult this element of proving not only the charges at issue but also a course of conduct, a pattern -- sets a very high goal. It's atypical," he added, saying it's not a "slam-dunk" case. "More often, you'd see something like a prior conviction being the predicate that establishes the pattern. Here, they have to prove brand new additional fact patterns."

Farrow noted that Weinsten over the years "has been very effective in manipulating exactly this kind of a process."

"You've already seen in this case Harvey Weinstein's lawyers succeed in getting one of the charges against him dropped," he said. "You've already seen them launch attacks on the witnesses in the press. There was a confrontation yesterday where prosecutors were requesting a gag order so that Weinstein's lawyers would stop talking to the press."

"They didn't get it," he added, "but the judge did chastise Weinstein's lawyers and say, 'You've got to stop impugning the witnesses.'"

Weinstein is also under investigation for sexual assault in London and Los Angeles. Farrow said prosecutors in the latter city, which announced new charges Monday, seems to be taking "a similar approach" to their Manhattan counterparts.

"They certainly at last will be bringing in other character type witnesses," Farrow said. "They've asked some of my sources, for instance, would they be willing to testify, even though those sources didn't have instances in their allegations that took place in that jurisdiction."

Weinstein's New York trial began Monday, more than two years after bombshell reports from reporters at The New York Times and from Farrow in The New Yorker.