Gary's stage adaptation of the 2003 film Calendar Girls, The Girls, debuted at London's Phoenix Theatre on Monday (20Feb17) and Howard had apparently planned to attend.
According to editors at British newspaper The Sun he was absent as his wife Katie Halil was ready to go into labour.
"Howard was upset to have missed the show last night but he needed to be on standby for the arrival of his fourth baby," an insider tells the paper.
"He has always been a rock for his bandmates and Gary completely understood why he couldn't be there. Everyone is really excited for the birth - it is just a matter of time now."
According to the publication the new arrival may put the planned Take That reunion on Saturday (25Feb17) night, for the final of Gary's TV talent show Let It Shine, in jeopardy.
Original Take That member Robbie Williams is set to rejoin the band for a one-off performance, though Jason Orange, who quit in 2014, won’t be there. Let It Shine sees hopefuls battle it out for a place in Gary’s next musical, The Band, which centres around Take That’s biggest songs.
Howard revealed he was to become a father for the fourth time in October (16), writing on Twitter, "Guess what? Me and katie are happy to announce we will have our 2nd child in Feb. Yay! No more sleep for another 2 years!"
Representatives for the Back for Good singer did not respond to requests for comment.