Muscle cell growth and the distribution of water and electrolyte in human pregnancy

Ten normal pregnant women had muscle composition analyses (rectus abdominis) carried out at 39-40 weeks of pregnancy. Water, chloride (Cl), chloride space (ECV), non-chloride space (ICW), potassium (K), sodium (Na), magnesium (Mg) and zinc (Zn) determinations were carried out. Analyses for DNA (cell number), protein: DNA ratio (cell size), RNA and collagen were also performed. Similar analyses were performed on uterine muscle and placentae before and after perfusion with Earle's solution. Data from pregnant patients were compared with similar estimations carried out on rectus abdominis samples from 13 non-pregnant subjects undergoing gynaecological procedures. Muscle tissue and predicted muscle mass (MM) (which constitutes 40% of body weight) demonstrated that the gain in body K was due to the products of conception, that ICW decreased per unit weight in muscle (8%), ECV increased (41%) without a radical change in muscle water content (2%). Overall a 6 l gain in ECV and a 2 l gain in ICW can be accounted for during pregnancy. The results of this study indicate that added hydration excluding the products of conception (placenta, infant, uterus) is mainly extracellular. Intracellular Na concentration decreases (50%) and it is speculated that the cation gap is made up by H+ in the presence of extracellular alkalosis. Muscle cells diminish in size but cell number per gram is constant. Zinc content (Zn/DNA) decreases. Previous experimental work suggests that MM increases by about 10% during pregnancy and this information has been included in considerations but it remains to be shown to what extent total muscle cell numbers increase and as to whether such increased muscle growth remains following pregnancy.