The uterus is normally the size of a pear. According to the Mayo Clinic, however, by the end of pregnancy it will take up the space between your pubic bone and your ribcage. Uterus changes begin as early as two to three weeks into a pregnancy, at or around the time fertilization takes place. The earliest uterus changes in pregnancy are the ones that prepare for the baby's future growth.
Corpus Luteum Development
Almost immediately after an egg is fertilized, a structure called the corpus luteum develops. The corpus luteum surrounds the growing embryo in order to secrete progesterone, one of the main pregnancy hormones. This progesterone helps to keep the uterus from contracting as it normally would during a menstrual period. The corpus luteum encourages continued growth of the lining that covers the walls of the uterus, so that when the embryo attaches in a few days it will receive adequate nourishment.
When the embryo implants in the uterine wall, roughly six to 12 days after the egg is fertilized, tiny finger-like projections develop. These projections eventually become the placenta, which supports and nourishes the growing embryo. The placenta is also responsible for producing both estrogen and progesterone, which will cause continued changes in the uterus throughout pregnancy.
Blood Vessels Enlarge
The blood vessels in the uterine lining will start to enlarge around the three to four week mark, which occurs approximately one to two weeks after fertilization. These blood vessels help the uterine lining to become thicker, which is necessary to support and nourish the embryo as it grows.
At around the four week mark, the cervix (which is the opening at the bottom of the uterus) starts to change in color and texture. In fact, this will be visible during a pelvic exam. At this early point you may still be unaware that you are pregnant. Around week five, the cervix color will be slightly bluish as it continues to thin. By week seven, approximately five weeks after fertilization, a mucous plug forms in the cervix to seal the uterus from outside contamination, keeping the embryo safe.
The uterus, which is normally about the size of an average pear, starts to become larger around the five week mark (approximately three weeks after fertilization). While it will still not be large enough to see or feel from the outside, it will start to lean on the bladder already. Some women find that their need to urinate increases during this time due to the extra pressure. For some, urinary incontinence may develop. This is usually temporary, and often disappears by week 12, which is when the uterus moves out of the pelvic cavity.