If sex date is 21 March can a baby born on 29th of August?? After five months of pregnancy?? OR what is minimum premature child birth after sex??

Human Gestation is NINE months, although babies can survive after as little as 5 months, but not usually.

Also conception doesn't always occur the same day of having sex... it can be as much as 7 days later. You have no way of knowing exactly when. Further shortening that time.

Sounds like there is more to this story, like there might have been more than one sex partner involved.

I think you were already pregnant in March. How premature did the doctors say your child was, and what did they do about it?

Yes, a baby can be born at 5 months gestation. It's called premature birth. Until 24 weeks the chances of survival are slim and increase with each week.

To help you, we really need more information. You haven't given us anything to go on.

If the baby was born, it would need special care and be obviously born early.

If this is about a woman claiming that a certain man is the father, I wouldn't believe it without a lot of documentation from a hospital. That baby, if he or she survived, needed weeks of intense care in a hospital.

IF it was a natural full term birth then no. There hasn't been any evidence that a full term baby can be born in that small of a time. If someone is trying to tell you that then they're trying to pull something on you. Be careful.