The wife of Japanese Emperor Akihito's younger son, who is pregnant with a possible heir to the throne, has signs of problems with the placenta, Kyodo news agency said on Tuesday, quoting the Imperial Household Agency.

The Japanese imperial family poses for a New Year photograph at the Imperial Palace in Tokyo December 16, 2005. (Front from L to R) Crown Princess Masako, Crown Prince Naruhito, Princess Aiko, Emperor Akihito, Princess Kako, and Empress Michiko. (Rear from L to R) Princess Mako, Prince Akishino and Princess Kiko. Picture taken December 16, 2005. [Reuters]

Kyodo said that Princess Kiko, 39, whose baby is due in late September, had symptoms of placenta previa, a complication in which the placenta becomes implanted at a location lower than normal in the uterus.

No male has been born into Japan's imperial family since 1965 and the possibility that Kiko might bear a son halted plans to revise a 1947 imperial succession law to give equal women equal rights to inherit the throne. That would have cleared the way for Princess Aiko, the daughter of Crown Prince Naruhito, to become Japan's first reigning empress since the 18th century.