In what week could a belly piercing become infected?

A "Navel" piercing can become infected at any time during the healing process. It generally takes a full year for a piercing to be considered "seasoned", this is the point at which it is considered fully healed. Contrary to the misinformation provided by the internet, all piercings take much more time to fully heal than what is indicated by piercers and novices in the industry.

Any time the piercing is subjected to a direct impact, snagging, pulling, improper aftercare or the jewellery is changed too soon during the initial healing period the piercing can become infected. Only proper common sense aftercare can prevent a piercing from becoming infected. Proper personal hygiene and washing the piercing daily will ensure a problem free piercing. Ensuring you are not putting "garbage" into your piercing, this would be cheap junk jewellery from mall vendors and locations that don't actually provide body piercing services. Look after your piercing, wear good quality body jewellery and don't touch the piercing with dirty hands, these are the steps to an infection free piercing.