Princess Kiko, the 39-year-old wife of Emperor Akihito's second son, is scheduled to give birth by a Caesarean operation on Wednesday following pregnancy complications.
The birth of a boy would almost certainly derail debate on revising Japan's males-only imperial succession law to let women take the throne.
''Before Kiko's pregnancy there was momentum toward change, but if a boy is born, enthusiasm will diminish,'' said Miiko Kodama, a professor at Musashi University in Tokyo.
Japanese tabloid media, never reluctant to probe celebrity secrets, have already decided that the royal baby is a boy.
In an article titled ''Countdown to Princess Kiko's Childbirth'', the weekly Shukan Bunshun wrote last week that Kiko's husband, Prince Akishino, had told a friend their third child would be a boy. But the magazine added nothing was certain.
The Imperial Household Agency has declined to comment on baby's gender, saying Kiko and Akishino, 41, who have two daughters aged 14 and 11, did not want to be told ahead of time.
No boys have been born into Japan's imperial family since Akishino in 1965, and Prime Minister Junichiro Koizumi had planned to enact legislation to give women equal rights to inherit the throne to avert a succession crisis.
The change would have put 4-year-old Princess Aiko, the only child of Crown Prince Naruhito, 46, and Crown Princess Masako, 42, next in line to the throne after Naruhito.
Kiko's pregnancy prompted the government to shelve the plan, which was opposed by conservatives eager to maintain a males-only tradition they say stretches back more than 2,000 years.
Among those is Chief Cabinet Secretary Shinzo Abe, widely tipped to succeed Koizumi later this month.
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