The uterus during early pregnancy undergoes rapid changes. The changes include growth and increased blood flow to support the growth of the embryo. These changes are responsible for some of the common symptoms of early pregnancy.
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 adapts to the pelvis and is protected by the pubic bone. By the end of pregnancy, it will have grown to about 1,000 times its usual size. The uterus is made of smooth muscle tissue lined with glands.
One change in the uterus during early pregnancy is size. As early as the third week of pregnancy, or one week of gestation, the uterus begins to grow. This growth puts pressure on the bladder and causes more frequent urination. The uterine growth can also cause cramps, similar to those of a menstrual period. Severe cramping, especially if accompanied by back pain and increasingly heavy bleeding, may indicate a miscarriage.
The uterus during early pregnancy gains more blood vessels. This allows food to pass to the baby and the placenta. Blood flow in existing blood vessels also increases. The elastic tissue in the uterine walls also increases.
The muscle fibers that make up the uterus soften due to the actions of the hormones estradiol and progesterone. This change of the uterus during early pregnancy allows for future growth. It also allows for greater contracting power which is then needed to expel the fetus.
The ligaments that support the uterus stretch to accommodate the growth. They also increase in diameter. This is sometimes felt as sharp pains in the lower abdomen.
Mild "practice" contractions, called Braxton Hicks contractions, begin as early as the sixth week of pregnancy. This action of the uterus during early pregnancy cannot be felt until much later in the pregnancy and can cause false labor. These contractions are important in preparing the uterine muscle for delivery.
The glands of the uterus increase secretions. These form a plug of mucus in the cervix, the opening of the uterus. This buildup of mucus prevents bacteria from entering the uterus from the vagina. It is expelled up to several weeks before delivery.
An unhealthy or abnormal uterus is sometimes discovered when a pregnancy occurs. If there has been previous uterine surgery, adhesions may develop. This can present with extreme pain as the uterus grows. Fibroids, which are benign tumors of the uterus, can be discovered during ultrasound examination. They usually do not cause problems during pregnancy, but they should be watched closely.