Pregnancy lasts for nine months, and is commonly divided into three trimesters, each covering a period of three months.
During the first trimester (the first three months) the structure and form of the foetus are developed. This is the most critical stage of the pregnancy. The second trimester is involved with the growth of the foetus while the third trimester is the maturity of the foetus.
A baby born at any time in the third trimester has a chance of survival, but the later the better.