Duration of pregnancy and Trimester of pregnancy

Duration - 

The duration of pregnancy has traditionally being calculated by clinicians in term of
                                     10 lunar months 
                                               or
                           9 calendar months and 7 days 
                                               or
                                          280 days
                                               or
                                          40 week
calculated from 1st day of the last menstrual period. this is called
menstrual or gestational age. But, fertilization usually occurs 14 days
prior to the expected missed period and in a previously normal cycle
of 28 days duration, it is about 14 days after the first day of the period.
Thus, the true gestation period is to be calculated by subtracting
14 days from 280 days i.e. 266 days. This is called fertilization age 
or ovulatory age and widely used by the embryologist.

Trimester -    Pregnancy is divided into 3 trimester-

                     1st Trimester -  01-12 week
                    2nd Trimester - 13-28 week
                  3rd Trimester -  29-40 week