It depends if she had copulated with someone, then it is possible to get pregnant. If they are regular in having a menstrual period, she needs to wait for three months to confirm it. Sometimes it is common for those women who have the irregular menstrual periods, so it is not a sign of being pregnant.
Delayed periods cause pregnancy, stress, sudden weight loss or overweight, hormonal imbalance, and polycystic ovary syndrome. However, you need to consult a doctor if the irregularity is persistent and has no apparent cause.