Hi, and thanks for your medical question on eHealth forum!
If you did not ejaculate inside then chances of pregnancy are less. If this was her first time, then the hormonal changes caused by sex could have delayed the periods. If periods do not come in another 2-3 days, it would be best to take a urine pregnancy test. At times periods can delay for a week-10 days without reason.
However, if they delay beyond that, please consult a gynecologist immediately. Hope this helps. Take care!