TOKYO - Japan's Princess Kiko, pregnant with a possible heir to the throne, donned a red and white silk "obi" sash in a traditional ceremony on Tuesday to pray for the royal infant's safe birth.

Japanese Princess Kiko, wife of Emperor Akihito's younger son Prince Akishino and who is pregnant with her third child, smiles as she leaves a general meeting of the Japanese Red Cross Society in Tokyo in this May 25, 2006 file photo. Japan's royal baby isn't due for weeks, but popular magazines, never shy about probing the secrets of the great and famous, have already decided that imperial family is about to welcome its first male heir over 40 years. [Reuters]

No male has been born into Japan's imperial family since 1965, and the possibility that Kiko - the wife of the emperor's younger son, Akishino -- might bear a son has halted plans to revise a law to give women equal rights to inherit the throne.

Kiko, 39, is expected to give birth through a Caesarean operation ahead of her late September due date because of a complication in her pregnancy, the Imperial Household Agency said last month.

The brief "Chakutai no Gi" ceremony is traditionally conducted in the ninth month of pregnancy on the Day of the Dog according to the zodiac calendar, because dogs are believed to have an easy time giving birth.

Akishino, clad in a morning coat, stood by and then tied the knot after Kiko's chief lady-in-waiting helped to wrap the sash - a present from Emperor Akihito and Empress Michiko -- over her ivory-coloured silk dress, an agency spokesman said.

Japanese weekly magazines have speculated that the baby is already known to be a boy, but the Imperial Household Agency says there will be no early announcements, not least because the parents do not want to know the child's sex ahead of time.

Plans for Crown Prince Naruhito and Crown Princess Masako to take a rare overseas holiday from August 17-31 have sparked speculation that Kiko's baby may be born then to spare Masako the strain of the celebrations that would follow the birth of a boy.

Naruhito and Masako, a Harvard-educated former diplomat who has been suffering from a mental illness due to the stress of adapting to royal life including pressure to bear an heir, have one child, 4-year-old Princess Aiko.

Aiko cannot inherit the throne under the existing law, which states that only males descended from an emperor can ascend the throne.