Seeking medical care within 72-120 hours is highly recommended for a number of reasons.
First and foremost, seeking medical attention can uncover physical harm done by the person who sexually assaulted you that may not be evident. An exam could also prevent sexually transmitted infections or diseases and pregnancy.
Evidence collection is most effective during the first 72 hours after an assault takes place.
To preserve evidence collection, victims should not shower, bathe, douche, eat, drink, smoke, clean up, urinate, or change your clothes. If you have already changed, place the clothes that you were wearing in a paper bag and bring them to the hospital. If you must urinate let the nurse know as soon as possible so she can collect the specimen. If you have already done any of these things, evidence may still be collected. Don't assume that all evidence has been destroyed. You may wish to take an extra set of clothes with you to the hospital if you are wearing the clothing you had on at the time of the assault. The clothes you were wearing at the time of the assault may be collected as evidence.