What can be the possible reasons if a girl's periods are delayed by almost one month? can obesity be a reason? (girl is around 23 years old and surlely not pregnant?

A pregnancy test should be done to make sure she is not pregnant since that is a common reason for a delayed period. If that is negative, then it typically means there was no ovulation. This can happen due to obesity but there are other reasons, like stress, increased exercise, hormone imbalance, etc. Best of luck to you!!

Agree with Dr. Ferranti. Menstrual irregularities are not uncommon and anovulatory cycle is a common cause. If you miss more than two periods it would be time to see your doctor. If you do not wish to be pregnant, use contraception all the time. You may consider implanted contraceptive or an IUD. Consult this site. http://www.nichd.nih.gov/health/topics/menstruation/conditioninfo/Pages/causes.asp