Photographs seen on North Korean TV showed Ri Sol-Ju wearing a neatly-fitting two-piece skirt suit in contrast to the loose-fitting clothes she was seen wearing in December, which suggested she was heavily pregnant, the Daily Mail said.
Adding to the speculation that Ri has given birth was her recent slimmed-down appearance at an official event, where an all-woman band sang Johnny Mathis' song "When a Child is Born".
"Before-and-after" photos were also run on South Korean TV and in newspapers in Seoul.
"The bulging stomach has gone down - has Ri Sol-Ju given birth," asked the Dong-A Ilbo newspaper.
The caption in another South Korean paper, the Chosun Ilbo, read: "Ri Sol-Ju with her tummy reduced in 11 days. Has she come out right after childbirth?"
A South Korean government official told the Yonhap news agency that Ri "appeared to have already given birth based on analysis of the TV images".
North Korea has not yet made any official announcement.
Even when Kim Jong-Un was married, state media revealed it quite late, long after pictures had emerged of a stylishly-dressed young woman accompanying the new leader at official events.
According to South Korean media, citing intelligence reports, the couple were married in 2009 and already have one child. But that has never been confirmed.
