女の子しか産まれない？

[Will]

女性主人公にして、結婚後、出産しました。
男兄弟二人も結婚して、出産しました。
女兄弟は未婚。

あわせて１０人ほど産まれましたが、全て女の子です。バグですか？

[TruXurT]

Used google translate to read your post. I think there is a separate Japanese forum, which may work better for you.

Assuming you don't have any mods installed, that could affect the base game, the sex of the children is random. The good news is that the sex is decided at birth, so if you have a save game right before child is born and re-load it, you may have a boy next time around. It also works if you are particular about the traits for the child. You may need to re-load the game few times, so be patient.

The pregnancy in Bannerlord last for about 35 days, so start saving the game frequently when you are approaching that time.

[Clovis Sangrail]

Gestation period is 1 season plus 15 days from the time you get the announcement the mother is pregnant.

Sex isn't the only thing determined at birth. Other things -- twins, stillborn, mother dying in childbirth, and to some extent, culture.