Princess Kiko, whose pregnancy could be the answer to Japan's imperial succession dilemma, has performed a ceremony to pray for a safe delivery.
Princess Kiko already has two daughters
The princess, who is due to give birth in September, wore a red and white silk outfit for the ceremony in Tokyo.
The pregnancy is attracting enormous attention from the Japanese public, many of whom are hoping for a boy.
Current law allows only males to ascend the throne, but the emperor's children have yet to produce a male heir.
Crown Prince Naruhito and his wife, Princess Masako, have a daughter, Aiko. The Emperor's third child, Princess Sayako, married a commoner and so her children would not be able to ascend the throne.
Princess Kiko, 39, who is married to the crown prince's younger brother, Prince Akishino, already has two daughters.
Currently only males can ascend the Japanese throne
Emperor Akihito has two sons, Naruhito and Akishino
If Naruhito died without a male heir, Akishino succeeds
His wife, Kiko, is now pregnant
Princess Sayako married a commoner so her children cannot ascend throne
The announcement of her pregnancy in February put on hold plans by Prime Minister Junichiro Koizumi to introduce legislation allowing women to ascend the throne.
Last month, it was announced that she was likely to give birth by caesarean section due to complications with the pregnancy.
The princess wore an elaborate outfit for the ceremony, including a ceremonial belt from the imperial couple.
The ritual was performed on the Day of the Dog in the lunar calendar, because dogs are thought to have easy deliveries.
Earlier, it was announced that Prince Naruhito and Princess Masako will take a two-week holiday in The Netherlands at the end of August.
This would be a rare overseas trip for the crown princess, who is reported to have suffered from depression for the last few years.