The increase in the uterus is due to changes in the average muscle layer - myometrium. The active division of myometrium muscle cells, the formation of new muscle fibers, provides growth of the uterus in the first half of pregnancy. In parallel, the formation of new fibers, they are lengthened, and thickened. The size of the uterus during pregnancy, albeit initially not so much, begins to change already in the first weeks - by the 4th week the uterus has dimensions similar to the size of a chicken egg. In this case, the shape of the uterus is often pear-shaped - asymmetry arises from the protrusion of one of the corners of the uterus, which is explained by the growth of the fetal egg. As you grow, the fetal egg fills the entire uterine cavity and the protrusion disappears. By the 8th week, the size of the uterus during pregnancy is already approaching the size of the goose egg, that is, the uterus is doubled, thus acquiring a globular shape. And already on the 10th week of pregnancy the uterus increases three times, at the 12th week - four. Thus, the size of the uterus during pregnancy changes, as they say, "by leaps and bounds". So, on the 12th week, the size of the uterus is similar to the size of the head of a newborn baby, and the bottom of the uterus reaches the upper edge of the lonnoe articulation.
If before the 20th week, the size of the uterus during pregnancy changes due to the division of myometrium muscle fibers, their thickening and lengthening, then after the 20th week the uterus increases only due to stretching and thinning of the walls. So, if before pregnancy the thickness of the walls of the uterus is about 4-5 centimeters, then by the end of pregnancy this indicator decreases to 0, 5-1, 5 centimeters. The same applies to other indicators of the size of the uterus: its length before pregnancy is about 7-8 centimeters, in the prenatal period, the uterus lengthens to 37-38 centimeters. The width of the uterus, which before the size fluctuated within 4-6 centimeters, reaches 25-26 centimeters by the end of pregnancy. The volume of the uterine cavity, in turn, increases 500 times in the last - ninth - month of pregnancy. Separately it is worth remembering the changes in the parameters of the weight of the uterus. So, if before the onset of pregnancy the body weighed 50-100 grams, then during the prenatal period the weight increases correspondingly to the increase in the size of the uterus during pregnancy: by the end of pregnancy this figure is on the average 1000-1200 grams - this is without the fetus and membranes.
Changes in the size of the uterus during pregnancy, the woman practically does not feel - the process of growth of the uterus occurs gradually and smoothly. And, nevertheless, the stomach grows just due to the increase in the size of the uterus during pregnancy, while the uterus as if "rises" - if, for example, on the 24th week the bottom of the uterus is at the level of the navel, then by the 28th week the bottom of the uterus is determined by 2-3 fingers above the navel. The highest position in the abdominal cavity reaches the uterus around the 38th week of pregnancy, after which, in a short time, the baby slips down. Thus, the baby takes the occipital presentation, getting ready for birth, and the uterus descends to 3-5 centimeters.
By the end of pregnancy, the uterus has the shape of a muscular sac, whose length is about 35 centimeters with walls no more than 1.5 centimeters thick. The baby is already ready to please mom and dad with a birth, which is due to contraction of the uterus and expulsion of the fetus outside.