The gestation period is actually a fancy term to say how long a woman is
pregnant and when she is going to have a baby. So, when it comes to
human females, this period of pregnancy lasts from 38 to 42 weeks, if
there aren't any complications. If a child is born before week 37, then
that would be a premature baby. When it comes how the age is determined,
well, it starts from the day the baby is born - that is its first day
"alive."
The length of gestation starts on the first day after the last period and lasts until the end of the pregnancy/until the baby is born. The age is determined by counting the days since contraception.
