Three months into the writers' strike, the broadcast networks’ primetime ratings have been holding up, seeing almost no decline as a direct result of the contract dispute.

That's about to change.

February sweeps, which began last night, is expected to see ratings tumble.

Typically, sweeps is a month in which schedules are chock-a-block with original fare, such as specials and episodes of top shows featuring big-name guest stars.

But this sweeps will be chock-a block with untested new midseason series, reality shows and repeats as the networks’ storehouses of original episodes of scripted series finally run out.

That spells trouble.

In all, ratings this February could tumble as much as 20 percent from last year in adults 18-49, predicts Steve Sternberg, executive vice president of audience analysis at Magna Global, the giant media buying agency.

Half of that decline will be the result of natural audience erosion as more viewers move to cable, but the rest will be directly attributable to the strike.

Though Sternberg says there will be only slightly more repeats than last year, they will take a toll. During last February's sweeps period, ratings for repeats were 47 percent lower than for originals among 18-49s, according to Magna.

"But the real impact will be having fewer original episodes of established hit series," he says.

An interesting point the report makes is that the networks have actually been increasing the number of repeats that run in sweeps in recent years.

This longer-term trend is the direct result of the longer television season. Back in the 1990s, the broadcast season was stretched out from September through April into May, the purpose being to be able to air original episodes during the May sweeps period, sweeps being the time when local audience data was gathered by Nielsen in order to set local TV ad rates.

More original episodes in May meant higher ratings for affiliates, which meant more ad revenues to those stations.

But the longer season meant the networks had to sprinkle more repeats throughout the season.

Repeats in the 1990s accounted for about 15 percent of primetime hours during non-sweeps months, compared to 40 percent now.

The repeats had a way of falling in certain months, notably December and January, especially around the holidays, when viewing levels go down. Reruns in January typically make up more than a third of primetime lineups.

But over time, February too became dotted with reruns. Back in the 1980s and 1990s, just 8 percent of February primetime was given over to repeats, but by last February that figure had risen to around 20 percent.

Part of this is due to the sweeps simply becoming less important as Nielsen began replacing the old system of gathering local audience data with the electronic people meters in major markets. Now data flows in daily.

But the LPMs are not the only reason, notes Sternberg.

“The major factor is not really the LPM markets, although certainly the sweeps are less significant than they used to be,” he says. “It’s that the networks do not want to spend money on risky multi-episode specials, combined with the fact that repeats of hit shows tend to do better than most new midseason shows."