For the first few weeks, the uterus maintains its original piriform or pear shape. But, as pregnancy advances, the corpus and fundus become globular and almost spherical by 12 weeks' gestation. Subsequently, the organ grows more rapidly in length than in width and becomes ovoid. By the end of 12 weeks, the enlarged uterus extends out of the pelvis. With this, it contacts the anterior abdominal wall, displaces the intestines laterally and superiorly, and ultimately reaches almost to the liver. With uterine ascent, it usually rotates to the right, and this dextrorotation likely is caused by the rectosigmoid on the left side of the pelvis. As the uterus rises, tension is exerted on the broad and round ligaments.
With the pregnant woman standing, the longitudinal axis of the uterus corresponds to an extension of the pelvic inlet axis. The abdominal wall supports the uterus and maintains this axis, unless the wall is lax. When the pregnant woman lies supine, the uterus falls back to rest on the vertebral column and the adjacent great vessels.
Beginning in early pregnancy, the uterus contracts irregularly, and these may be perceived as mild cramps. During the second trimester, these contractions can be detected by bimanual examination. In 1872, J. Braxton Hicks first brought attention to these contractions, which now bear his name. hese appear unpredictably and sporadically and are usually nonrhythmic. Their intensity varies between 5 and 25 mm Hg (Alvarez, 1950). Until near term, these Braxton Hicks contractions are infrequent, but their number rises during the last week or two. At this time, the uterus may contract as often as every 10 to 20 minutes and with some degree of rhythmicity. Correspondingly, uterine electrical activity is low and uncoordinated early in gestation, but becomes progressively more intense and synchronized by term (Garield, 2005; Rabotti, 2015). This synchrony develops twice as fast in multiparas compared with nulliparas (Govindan, 2015). Late in pregnancy, these contractions may cause some discomfort and account for so-called false labor.
The delivery of most substances essential for fetal and placental growth, metabolism, and waste removal requires the placental intervillous space to be adequately perfused (Chap. 5, p. 94). Placental perfusion depends on total uterine blood low, but simultaneous measurement of uterine, ovarian, and collateral vessels is not yet possible, even using magnetic resonance (MR) angiography (Pates, 2010). Using ultrasound to study the uterine arteries, uteroplacental blood low has been measured to increase progressively during pregnancy-from approximately 450 mUmin in the mid trimester to nearly 500 to 750 mU min at 36 weeks (Flo, 2014; Wilson, 2007). hese measures are similar to uterine artery blood flow estimates ascertained indirectly using clearance rates of androstenedione and xenon-133 (Edman, 1981; Kauppila, 1980). These values also mirror older ones-500 to 750 mUmin-obtained with invasive methods (Assali, 1953; Browne, 1953; Metcalfe, 1955). Logically, such massively increased uteroplacental blood flow requires adaptation of the uterine veins as well. The resultant increased venous caliber and distensibility can result in uterine vein varices that in rare instances may rupture (Lim, 2014).