All going well, the high walls of the home to the world's oldest hereditary monarchy will soon echo with the cries of an imperial bundle of joy. Crown Princess Masako, the commoner who married Crown Prince Naruhito, the son of Emperor Akihito, is expecting their first child at the end of the month.

Like everything that happens behind the gates of Tokyo's Imperial Palace, the details of the pregnancy - not least its inception - are shrouded in secrecy.

Rumours abound that the baby is the product of in-vitro fertilisation. After eight years of marriage weighed down by the burden of producing a male heir to an institution that supposedly traces its lineage back to 600 BC, the desperate couple are thought by many to have called on fertility specialists.

No big deal, perhaps, except the Japanese have not been able to read this in their own press. It was a British journalist, Richard Lloyd Parry of the London Independent, who last April put together fairly compelling evidence that the baby may well become the world's first test-tube emperor.

Speaking in public about the Japanese first family, except in the most reverential tones, can be a risky business. When Uwasa No Shinso - "the truth of the rumour" - one of Tokyo's racier magazines, wrote about the princess last year, pointedly omitting the honorific Sama before her name, two ultra nationalists beat the editor and subeditor into hospital for not showing respect.

"We're a bit more careful now," says Megumi Yui of the magazine. The editors were lucky. An Asahi journalist was shot dead by a rightist for a similar offence in 1987.

Such incidents help ensure the media rarely stray from the official line laid down by the shadowy Imperial Household Agency. Few, if any, reported that the Dowager Empress Nagako, who died last year, suffered from dementia, and the ultimate taboo - her husband's responsibility for the second World War - is seldom broached.

Almost 13 years since his death, Hirohito remains the central figure in the family's cast of characters, personifying why it remains deeply controversial. Revered as a god until US post-war occupiers stripped him of his divinity and tried to turn him into a European-style monarch, Hirohito remained a distant figure who stayed aloof from the debates that raged about his role in the war and the millions who died with his name on their lips.

Most accept that he was imprisoned by court officialdom and largely controlled by the militarists who surrounded him, but many resent that he neither abdicated nor spoke publicly about his motives.

"People of my generation will never forgive the emperor for staying silent about all those lives," says Takemitsu Ogawa, a doctor and pacifist who worked in occupied China during the 1940s.

This mute stoicism still characterises today's institution and its head, Emperor Akihito, allowing it to retain its enormous symbolic importance for the forces of authoritarianism and nationalism, which still bubble beneath the surface of Japanese politics.

"Many of us worry about the monarchy here, because they are what gives the nationalist right its respectable centre," says Dr Ivan Hall, a historian who sees a steady drift to the right in Japanese politics.

"It is the ultimate linchpin of the myth of Japanese uniqueness, because the emperor is supposed to be descended from the gods, and the lodestar for the most repressive ideas of racial superiority."

There were hopes that Oxford-educated Princess Masako, who as plain Masako Owada was a tough, independent career diplomat, might give the family a modern voice. These days, she can be spotted smiling sadly and carefully walking three steps behind her husband, which is where etiquette demand that she be. Far from being the Japanese queen of hearts, few people have heard her voice since 1993.

Her child will have a lot to live up to. Due to be born into a country struggling with economic and social problems, the baby is already being hailed by some as a balm for the troubled national soul. "A male heir will excite nationalists, who may well use the birth to expand their influence," says Hall.

Venerated by some, loathed by others, but mostly treated with indifference, he will spend the formative years of his life preparing to follow his 126 real and mythical predecessors as "symbol of the people and the unity of the nation". With the outside world filtered by retainers, he will have little idea about what the public thinks of him - the idea of a Japanese newspaper taking a survey of the family's popularity, as the Guardian does of the Windsors, is unimaginable.

At least he may have more fun than his great-grandfather Hirohito. Taken away from his mother at the age of three months, and forbidden from having friends, the young emperor had his only spontaneous fun during his stay with the British royal family, in 1921. "It was the most beautiful time in my life," he said afterwards.