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.
40 weeks is about 9 months and 1 week (9.2 months). Actual gestation (from conception to birth) is just under 9 months. Whether or not a baby is premature depends more on its size and weight than how many weeks along the mother is in the pregnancy. When referring to length of pregnancy, if a baby comes early (generally before about 36 weeks) it is best referred to as pre-term. This means that a baby could be pre-term but not premature.