Kiko, 39, is due to give birth in late September, but her doctors said last week she has symptoms of partial placenta previa, a condition in which part of the placenta drops too low in the uterus, and may have to give birth to her third child by Caesarean section.
During a medical examination Thursday, the first since the July 18 announcement of the problem, doctors said both Kiko and her fetus are "stable," said Imperial Household Agency spokesman Yasuo Moriyama.
Although she still has the placenta problem, "everything else is going smoothly," Moriyama said. "No other immediate problems have been found."
Palace doctors have said the baby may come earlier than expected, the AP reports.
The pregnancy of the 39-year-old wife of Prince Akishino, who is the second son of Emperor Akihito, has won national attention because the imperial family has failed to produce a male heir to the throne since 1965. The current law allows only males to reign.