During Book Three, there is a great deal of fuss over Kiyomori's Daughter having a son by the current Emperor; "now the exulting Heike imagined how wonderful it would be if she had a boy."
"On the first day of the sixth month... he performed the rite to change a girl in the womb into a boy."(pg.132)
To make sure that a boy was born many rituals were enacted, many pardons given, and many sacrifices made so that the spirits were calmed and the there was no bad Karma to cause the pregnancy any ill will. They Heike, and Japanese in general, would grant amnesty to exiles and those on death row in preparation of events like this all the time. "Precedent permitted a general amnesty to honor such an occasion. One had been declared in Daiji 2... when Taikenmon-in gave birth." (pg.141)
Many sutra readings occurred at over "twenty shines ...(and) sixteen temples"
After a great deal of praying, protection from evil spirits, gifts to many temples and many people, and great anxiety, Kiyomori's daughter delivered a prince.
Now, after three full episodes dealing with the necessity of a boy to be born, and all of the rituals performed to make it so, the reader would think that this was commonplace in ancient Japan. It was not.
It was only done in this fashion because of Kiyomori's specific circumstances. In most cases, aristocrats actually wanted daughters so that they could later marry those daughters to higher ranking aristocrats and gain a status boost. This progress would, hopefully, eventually lead to the situation that Kiyomori is in during Episode three: a daughter would give birth to a son who would grow up to be emperor. The maternal grandfather (in this case, Kiyomori) and uncles would serve as his advisors . Having this control over the government is a noble's dream, and thus Kiyomori strove for it, marrying up himself to put his children at an advantage in the future.
Shigemori's dying dream.
"The Tale of the Heike, translated by Royall Tyler, 2012
Another note of Family
Families in "The Tale of the Heike" are very important, they are the protagonist of many battles, feuds, and other affairs.
We see family names and the idea of brotherhood and staying with ones family throughout this novel.