can avert pregnancy
by three ways-preventing ovulation if it has not yet happened, thickening the cervical mucus making it difficult for sperms to ascend if ovulation has already occurred, making the endometrium hostile for implantation
if conception has occurred. It cannot do anything if implantation also has occurred. In your case, since she took the pill within the stipulated time, protection should have been high. To know exactly when the pregnancy happened, you can calculate back from the EDC. Hope this helps.