My girlfriend is a month late and still no sign of her period. what should we do?

Sexually active? If: Yes check pregnancy test. Reasons for missed/late periods: low body wt, obesity, marked wt ^ or v, over-exercise, endurance athletics, breast-feeding, pregnancy, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (polycystic ovary syndrome or asherman’s syndrome).