These are the first 13 weeks of the pregnancy. The baby develops the fastest during this period and becomes almost fully formed by the end of it. Women say this trimester is when they are most likely to feel tired, nausea, and breast tenderness.
The 2nd trimester (14th to 26th week)
This is from the 14th to 26th week of the pregnancy. During this trimester it becomes clear that the mother is pregnant. As well as weighing more because of the growing baby, the expanded uterus, the placenta and the amniotic fluid, the mother lays down extra reserves of fat. The baby continues to grow and develop. During the second trimester the mother will feel the baby's movements, as will others if they place their hands on the mother's stomach - sometimes movement may be observed without touching.
The 3rd trimester (27th to 42nd week)
This is from week 27 until the baby is born. In this trimester the baby will build up fat stores and continue growing rapidly. The baby's lungs will develop, as will his/her sense of hearing, taste and sight. The mother may experience backache and find it harder to get into the right position for a good night's sleep. The mother will also be urinating more often as the baby and everything around him/her presses against her bladder. She may also experience contractions that take place many days or weeks before the birth - they are called Braxton Hicks contractions - these are not the contractions of labour.