The uterus undergoes rapid changes during early pregnancy. Changes include growth and increased blood flow to support the growing embryo. These changes are responsible for some of the common early pregnancy symptoms.
In women who are not and have never been pregnant, the uterus is about the size of a small pear. The uterus is slightly larger in women who have been pregnant in the past or are currently pregnant. It fits into the pelvis, and is protected by the pubic bone. By the end of pregnancy, it will have increased to about 1,000 times its usual size. The uterus is made of smooth muscle tissue lined with glands.
One change to the uterus during early pregnancy is size. As early as the third week of pregnancy, or one week gestation, the uterus begins to grow. This growth puts pressure on the bladder and causes more frequent urination. Uterine growth may also cause cramping, similar to that during a menstrual period. Severe cramping, especially if accompanied by back pain and increasingly heavy bleeding, may signal a miscarriage.
The uterus during early pregnancy gains more blood vessels. This allows nourishment to pass to the baby and to the placenta. Blood flow in the existing blood vessels increases as well. The elastic tissue in the uterine walls also increases.
Muscle fibers that make up the uterus soften due to actions of the hormones estradiol and progesterone. This change of the uterus during early pregnancy allows for future growth. It also allows for the greater contracting power later needed to expel the fetus.
Ligaments that support the uterus stretch to accommodate growth. They also increase in diameter. This is sometimes felt as sharp pains in the lower abdomen.
Mild "practice" contractions, called Braxton Hicks contractions, start as early as the sixth week of pregnancy. This action of the uterus during early pregnancy can not be felt until much later in pregnancy, and may cause false labor. These contractions are important for preparing the uterine muscle for labor.
The glands of the uterus increase secretions. These form a mucus plug in the cervix, the opening of the uterus. This build up of mucus prevents bacteria from entering the uterus from the vagina. It is expelled up to several weeks preceding labor.
Sometimes an unhealthy or abnormal uterus is discovered when a pregnancy occurs. If there has been previous uterine surgery, adhesions may develop. This may present with extreme pain as the uterus grows. Fibroids, which are benign tumors of the uterus, may be discovered during ultrasound examination. They usually do not present problems during pregnancy, but need to be closely watched.