A bastard is a child conceived outside of marriage or concubinage. Bastards are born with the mother's culture, religion, and caste.
Bastards who are identified as such are born with the 
trait, making them unable to inherit from either parent, and preventing them from passing on either parent's dynasty.
Bastards can be declared legitimate by either parent. A 
becomes a permanent member of the dynasty of the legitimizing parent, and is allowed to inherit from both parents. Muslims are not allowed to legitimize bastards and their Open succession does not exclude bastards.
Pregnancy
Pregnancies resulting in bastard children can be started by lovers, based on normal fertility mechanics, but with lower chances than for even secondary spouses. Women can also become pregnant in events. The most reliable way to become pregnant is with repeated tumbles (but not taking lovers) in the Seduction focus.
While unmarried
If an unmarried woman becomes pregnant, she loses 50 prestige and 50 piety, and the biological father is known at the time of birth.
While married
If a married woman secretly becomes pregnant with another man's child, the husband may search for the identity of the biological father. This event chain occurs during pregnancy, and determines whether the "known father" is the husband (making the child a secret "cuckoo bastard") or the biological father.
The husband's chance to suspect is 0% if 
, 100% if 
, and 25% otherwise. A husband who suspects then chooses whether to investigate; AI husbands who are paranoid or 
are very likely to do so. The investigation can succeed or fail in several ways; overall, the outcome depends on the intrigue skill and traits of both the husband and wife. Finally, if the investigation fails, he has the option to confront his wife, and is guaranteed to do so if paranoid. In this confrontation, the wife can choose to disclose her affair or leave him frustrated.
Thus, wives who wish to cuckold their husbands should marry trusting husbands, or at least ensure they have much better intrigue skill and traits. Wives who wish to produce known bastards should marry paranoid husbands. Wives who want maximum control should marry paranoid but low-intrigue husbands.
Birth
If the biological father is known at birth, he receives an event letting him choose between:
- Make the child a of his dynasty.
- Acknowledge the child as his . The child will temporarily be of the father's dynasty (until the child has children of his own via a patrilineal marriage) and can be legitimized by either parent later.
- Denounce the child. The child will no longer have a known father. The child immediately is the founder of a new dynasty (?), and can be legitimized by the mother later.
AI fathers who are 
and 
tend toward the higher options. 
Christians tend toward the lower options, as do all fathers when the child is a girl. Only men above 50 with no living sons are likely to choose the legitimize option.
If the father dies during the pregnancy, the unborn child gets claims on the father's titles. The child is temporarily a member of the mother's dynasty (until the child has children of his own via a patrilineal marriage), and can be legitimized later.
Legitimization after birth
Any known parent can legitimize a child by selecting the "Legitimize" interaction in the child's menu. The child must be a vassal or below of the parent, and the parent must be able to spend 20 piety. This brings the child permanently into the legitimizing parent's dynasty. This interaction is the only way for a mother to legitimize a child into her dynasty. AI parents do not do this.
Two rare events allow parents to legitimize adult sons, but without correcting the son's dynasty. In event 76015, a parent can legitimize a random son. In event 76020, a man with no sinful traits can ask his father for legitimization.
Creation of bastard dynasties
An illegitimate bastard who has a child will create a new dynasty. Thus, if you are going to legitimize a child, it is best to do so before they give you grandchildren.
If you cannot legitimize a child or decide not to, you might want to prevent them from marrying, in order to ensure claims do not pass out of your dynasty. One way to prevent them from marrying is by granting them a city. (Hindus may have difficulty using this method due to their caste system.) Christians and Zoroastrians can also be prevented from marrying, and even have their existing marriages annulled, by being granted a temple. With 
, Christians can also order their courtiers to take the vows.
Identifying biological fathers
There are two ways to identify the biological father of a bastard (including secret cuckoo bastards) after birth. The intrigue focus can be used to identify biological fathers of adults. The charinfo console command shows the names of biological fathers in tooltips. A character's appearance ("graphical culture") can be a hint that their known father is not their biological father.