It does matter. Menstrual cycles can vary from month to month, so it is hard to know exactly when intercourse can cause pregnancy especially for teenage girls.

Girls who are in their teens are more likely to have irregular periods, so determining when you are most likely to get pregnant, during your teen years, can be next to impossible. Also there are rare but well-documented cases of women becoming pregnant from vaginal intercourse at any time during the cycle so it is possible to become pregnant during your period and at all other times.

In general, a woman is most likely to become pregnant from vaginal intercourse during the six days that end in ovulation. However, she may become pregnant from intercourse during a total of eight days during her menstrual cycle - up to five days before ovulation, the day of ovulation, and two days following ovulation. So it important to use condoms at all times.