When a solid turns into a liquid it is called fusion, more commonly referred to as melting. When a material moves from one state of matter into another it is called a phase transition. Fusion is just...
Human pregnancy can be divided roughly into three trimesters, each approximately three months long. The first trimester is from the last period to the 13th week, the second trimester is from the 14th to 27th week, and the third trimester is from the 28th week to the 42nd week. In humans, birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks.
There's no official definition, but the trimesters are generally considered to be about three months each. At BabyCenter, you graduate to the second trimester the day you turn 14 weeks pregnant. The third starts the day you turn 28 weeks pregnant. Does pregnancy really last nine months? Of course pregnancy is nine months long, right? Well, not exactly.