State insurance regulators are on the watch for misleading marketing about short-term health insurance plans that don’t cover all the essentials required under the Affordable Care Act.

The Trump administration has made the short-term, limited duration plans easier to sell by extending the amount of time people can have them. The plans, which are offered by insurers like Anthem Inc. and UnitedHealth Group Inc., are intended to cover short gaps in coverage, such as when people lose their jobs. They’re usually cheaper because they tend to cover less routine care.

“Consumers are being bombarded with marketing for these products,” Pennsylvania...