The lack of the menstrual period is the first obvious sign of the pregnancy, but in some cases the period can be intact during the first couple of months of the pregnancy. But off periods not neccessary means pregnancy, there are certain conditions which cause this fenomenon. 

Pregnancy rapid test (or BHCG from blood)  shoud be performed to decide whether this is a pregnancy or not. 

The obstetrician can justify pregnancy by vaginal ultrasound only from the third week from the conception (5 weeks from the last menstrual period), but the tests should be positive after two weeks, when there is a delay of the period.

When pregnancy can be excluded,  the most possible causes are the following:

Stress (war, trauma, death in the family, work related problems etc..)

PCOS (Polycystic ovary syndrome)

Hyperprolactinemia (hypophysis adenoma or hyperfunction) 

Diets (heavy weit loss, vegan diet etc..)

Medications (antidepressants, drogs, hormonal treatments etc...)