Pregnancy is traditionally divided into three trimesters lasting approximately three months each. Since the average pregnancy actually lasts 40 weeks, not nine months, this can sometimes be confusing, according to Penn Medicine. Different doctors and organizations may measure trimesters slightly differently, but they typically last between 12 to 14 weeks each.
Definitions
The first trimester is the earliest third of a pregnancy and begins with the date of the last menstrual period. Since ovulation doesn't occur until approximately two weeks after the menstrual period begins, the first two weeks of the first trimester are actually before the baby has been conceived. The first trimester lasts until the end of week 13 of the pregnancy. Penn Medicine measures the second trimester as weeks 14 through 28, while the American Pregnancy Association defines it as lasting through the 26th week. The third trimester lasts from the end of the second trimester until birth, which occurs between weeks 38 to 42.
Symptoms
In the first trimester, pregnant women begin to exhibit the early signs of pregnancy, although they most likely don't have a visible bump yet. Morning sickness, breast tenderness and fatigue are common first trimester symptoms. In the second trimester, the abdomen becomes visibly larger, and the mother-to-be may begin to feel her baby move. Nausea and fatigue may subside in the second trimester. The third trimester brings back fatigue along with aches, soreness and swelling.
Fetal Development
During the first trimester, most of the organs form in the developing baby. The fetus grows to about 3 inches long during this trimester. In the second trimester, growth of fetus proceeds rapidly and it reaches about 14 inches in length by the end of this trimester. During the third trimester, the development of the lungs and brain are the main focus, and the baby grows to its final length of 19 to 21 inches. The final trimester is also the time when the baby puts on fat that will help sustain him after birth.
Tests
In the first trimester, after a pregnancy test has established the existence of the baby, common tests include blood tests to detect any underlying problems in the mother and an ultrasound to look at the fetus's development. In the second trimester, pregnant women often have screening tests for chromosomal problems such as Down syndrome and tests for gestational diabetes and preeclampsia. An in-depth ultrasound is usually given around the middle of the second trimester to assess growth and organ development and can also determine the sex of the baby if the parents want to know. The third trimester may involve monitoring of the baby's heartbeat and tests such as group B strep, which looks for bacteria in the vaginal tract.
Problems
Problems in pregnancy may occur at any time, but the risk of miscarriage is greatest during the first trimester. Gestational diabetes and preeclampsia do not generally show up until the second trimester. Problems that may occur in the third trimester include breech positioning, abnormal growth patterns, placental problems and premature labor.
