During pregnancy, the trimesters are the major milestones for the mother-to-be.  Rather than representing three three-monthly periods, they are of uneven length, and are defined by the physiology of fetal growth.  By convention, the trimesters date from presumed conception (two weeks after your LMP), and the first trimester represents the first twelve weeks of your baby’s fetal life.  The second trimester ends at 28 weeks, and the third trimester encompasses the rest of your pregnancy.   

During the first trimester, your body adjusts to pregnancy.  At the beginning you won’t look pregnant, and you may not feel pregnant either, but the activities of your hormones will soon start to affect you in various ways.  Your moods may change capriciously, your libido may decrease or increase and you prefer simpler, blander food.   

PHYSICAL CHANGES 

Your pregnant body has to work very hard to accommodate the developing embryo and the placenta.  Pregnancy induces a higher metabolic rate - between 10 percent and 25 percent higher than normal- which means that the body accelerates all of its functions.  Your cardiac output rises steeply, almost to the maximum level that will be maintained throughout the rest of the pregnancy.  Your heart rate increases too, and it will continue to do so until the middle of the second trimester.  Your breathing becomes more rapid as you now send more oxygen to the fetus and exhale more carbon dioxide. 

Owing to the action of estrogen and progesterone, your breasts quickly become larger and heavier and are usually tender to the touch from very early on.  Fatty deposits are increased, and new milk ducts grow.  The areola around the nipple becomes darker and develops little nodules called Montgomery’s tubercles.  Underneath the skin, you will notice a network of bluish lines appearing as blood supply to the breasts increases. 

Your uterus enlarges even in early pregnancy, but it cannot be felt through the abdominal wall until the end of the first trimester, when it begins to rise above the pelvic rim.  While it is still low in the pelvis, your uterus will increasingly press against your bladder as it enlarges, so that you will almost certainly find that you need to urinate more often. In addition, the muscle fibers of your uterus begin to thicken until it has become very solid.  However, you probably won’t notice any increase in your waistline until the end of this trimester.   (Conception, Pergnancy & Birth, by Dr. Miriam stoppard)