Oh, sweet irony.

Writers for the CBS sitcom "How I Met Your Mother" are being forced to work on the fly this season thanks to impending motherhood for its two lead actresses.

Both Alyson Hannigan and Cobie Smulders are pregnant and due this spring. While Hannigan and Smulders each are IS expecting their HER first child, you, faithful viewer, can expect that the pregnancies won't be acknowledged on screen with their "HIMYM" alteregos ALTER EGOS, Lily Aldrin and Robin Scherbatsky. The actresses told “Entertainment Tonight” last month that no plans have been made for their pregnancies to become part of the show's storylines.

Actresses working (or, in some cases, not working) through pregnancies is nothing new. Julia Louis-Dreyfus was put behind counters and in baggy clothes as Elaine Benes for much of a season of "Seinfeld" before giving birth to her second child in 1997. Mariska Hargitay took maternity leave from her role as Olivia Benson on "Law & Order: Special Victims Unit" in 2006, with Connie Nielsen temporarily filling in as a partner for Elliot Stabler (Christopher Meloni).

But TWO lead actresses pregnant at the same time on the same show? This is rare territory. For the most part, "HIMYM" taken a straight-forward, "nothing-to-see-here-folks" approach, keeping both actresses in baggy clothing or always seated in a booth at the gang's favorite bar, MacLaren's.

But they've had a little bit of fun with the situation too. During an episode last month titled "The Possimpible," Hannigan's baby bump made a cameo when, in a flashback scene, Lily won a hot dog-eating contest (Hannigan mentioned in the "ET" article that more hijinks could be in the works this season). No such baby bump moments yet for Smulders' character, but don't put it past "HIMYM" to work it in somehow.

In the meantime, you can enjoy "How I Met Your Mother" at a special time tonight, with a new episode airing at 8 p.m. on WOIO. A rerun will then follow at 8:30, the show's normal timeslot.

- Tom Valentino