When state TV showed Ri Sol-ju attending a memorial service for her husband's late father Kim Jong-il in December, she appeared heavily pregnant.
But more recent images of the first lady attending an official New Year's party showed her wearing a well-fitted skirt suit with no physical sign of pregnancy.
The rumours were fuelled by the official performance, which included an all-woman band singing a version of the Christmas favourite When A Child Is Born.
South Korean television and newspapers have run before-and-after photos with speculative captions.
"The bulging stomach has gone down... has Ri Sol-ju given birth?" asked the Dong-A Ilbo daily.
"Ri Sol-ju with her tummy reduced in 11 days. Has she come out right after childbirth?" ran the caption in the Chosun Ilbo daily.
The fact Kim Jong-un had a wife was only revealed last July when pictures emerged of a stylishly dressed young woman accompanying the new young leader at official events.
Pyongyang's state media confirmed her identity later that month in a rare move in the isolated and deeply patriarchal nation, which has rarely placed its first ladies under the spotlight.
According to intelligence reports cited by South Korean media, the couple were married in 2009 and already have one child, although that has never been confirmed.
Ri was described as coming from an ordinary family, with her father an academic and her mother a doctor.
She visited South Korea in 2005 as a cheerleader for her country's squad in the Asian Athletics Championships.