Princess Mako, the granddaughter of the Emperor and the daughter and sister of two prospective Emperors' is to marry in 2018.

"When Princess Mako marries, the number of Imperial family members will drop to 18, and that of female members to 13.

The news will likely further promote public debate over whether women should be allowed to retain Imperial status after marriage and whether women should be allowed to rise to the throne."

http://www.japantimes.co.jp/news/2017/05/16/national/princess-mako-granddaughter-emperor-engage-ex-classmate/#.WRtoUWjDHIU