The U.S. Department of Homeland Security is reminding law-enforcement officials of the increased use of female suicide bombers worldwide, and warning that women terrorists might hide explosives in devices "that mimic the look of a pregnant woman."

The unclassified DHS threat assessment, released Monday and obtained by NBC News, shows photographs of "pregnancy prosthetics," hollowed-out devices that could hold explosive devices. The report says "female suicide bombers have used devices that make them appear pregnant to hide explosive devices."

The DHS report states that terrorists are using female bombers more frequently, in part because these attacks generate "more widespread media exposure." It adds: "Continued use of female suicide bombers-the most recent example being a 3 February 2008 attack at the main railway station in Colombo, Sri Lanka-indicates that terrorists judge this tactic as effective in increasing defenses and thwarting security measures."

The report, marked "For Official Use Only," is one of dozens of threat assessments issued jointly throughout the year by DHS and the FBI. The authors are quick to mention that there’s no immediate threat to the U.S.

"DHS and the FBI have no specific, credible intelligence indicating that terrorist organizations intend to utilize female suicide bombers against targets in the Homeland," they write.

The report ticks off several recent attacks by women bombers in Iraq and Sri Lanka, and the recruitment of female suicide bombers in Chechnya, India, Pakistan, the Palestinian territories and Turkey.

Why the increase in women bombers? "DHS and the FBI assess that female suicide bombers may have an advantage over their male counterparts in accessing targets. The means to conduct a suicide attack vary widely, but a key element in maximizing the lethality of a suicide bombing is the bomber’s ability to get close to the target," the report states. "Female suicide bombers may carry explosives devices in a backpack, briefcase, or purse, or hidden in an article of clothing worn close to the body, such as a vest, belt, long socks, or jacket."