This is a clear case of adultery. Section 497 in The Indian Penal Code defines adultery as:
Adultery.—Whoever has s*xual intercourse with a person who is and whom he knows or has reason to believe to be the wife of another man, without the consent or connivance of that man, such s*xual intercourse not amounting to the offence of rape, is guilty of the offence of adultery, and shall be punished with imprisonment of either descripttion for a term which may extend to five years, or with fine, or with both. In such case the wife shall not be punishable as an abettor.
However adultery is no more a crime now, but, it can be used as a ground for divorce.
Adultery as a ground for divorce in India has been defined under Section 13(1) of the Hindu Marriage Act, 1955, as the act of having voluntary s*xual intercourse with a person who is not the spouse of the respondent. Hence, it becomes essential for the petitioner to prove that she/he was indeed married to the said respondent and that the respondent had voluntary s*xual intercourse with a person other than him/her.
You and your spouse are still married even if the case is pending in the court. If you or your spouse has a s*xual relationship with anyone else during your divorce case , it can be considered adultery. It can affect your divorce in the same way as adultery in the marriage.
Adultery is when a spouse has a s*xual relationship outside the marriage. If you're getting a divorce and adultery has taken place, it can have a significant impact on the divorce.
All states require you to give a reason for your divorce. Most states allow you to get divorced simply because you and your spouse no longer get along. Some states still require one spouse to allege fault, or that one spouse has acted in a way that justifies a divorce.
In states where fault is required or allowed, adultery can be the reason for your divorce. Proof of adultery may change the amount of child support and alimony a spouse receives. The spouse who was not at fault may also receive more of the household property in the divorce settlement
The spouse who wants to file a divorce petition has to substantiate the statements with proper evidence. The Indian Courts time and again had stressed that adultery has to be proven beyond reasonable doubt.
In a divorce, if you are claiming that your spouse has committed adultery, you must prove this to the court. There are two ways to prove adultery:
Direct Evidence. This is the best kind of proof. It can be a witness, a picture or some other proof that clearly shows the adultery. Direct evidence can be very difficult to get.
Circumstantial Evidence. If you do not have direct evidence, you will need to prove adultery in a different way. Circumstantial evidence may be used instead. You and your attorney will need to give the court information about your spouse. This information must show that your spouse had the opportunity and the interest to have a s*xual relationship outside your marriage. Adultery may be proven if, based on your evidence, it seems reasonable to the court.
In your case, baby born with the other person than husband can be the best evidence.
Hope this helps