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The first trimester begins with the start of gestational age as described above, that is, the beginning of week 1, or 0 weeks + 0 days of gestational age (GA). It ends at week 12 (11 weeks + 6 days of GA) [4] or end of week 14 (13 weeks + 6 days of GA). [36]

The birth rate in a period is the total number of live births per 1,000 population divided by the length of the period in years. The number of live births is normally taken from a universal registration system for births; population counts from a census, and estimation through specialized demographic techniques.

An average gestational period (duration of pregnancy from the first day of the last menstrual period up to delivery) is 280 days. On average, this is 9 months and 6 days. Gestational age estimation [ edit] A commonly used estimate of gestational age in weeks is (as described by Verburg et al. [2] ):

A probability distribution is a mathematical description of the probabilities of events, subsets of the sample space.The sample space, often denoted by , is the set of all possible outcomes of a random phenomenon being observed; it may be any set: a set of real numbers, a set of vectors, a set of arbitrary non-numerical values, etc.