During pregnancy, weight gain occurs as a result of hormonal changes with increased estrogen and progesterone synthesis. Initially, blood volume increase and uterine muscle weight dominate. From the 27th – 40th week of pregnancy (3rd trimester/third trimester) – weight is determined by the fetus, the mother’s fat storage and the increase in tissue fluid. The volume of total body water increases by about 8 liters during pregnancy, with most of the mother’s water gain occurring in the extracellular space (ECM). Plasma volume also increases during pregnancy, nearly doubling blood volume toward the end of pregnancy. Plasma volume increases by 1,250 to 1,500 milliliters, with the maximum value being reached at about the 34th week of pregnancy. In contrast, the amount of fluid in the red blood cells (erythrocytes) increases by only 320 milliliters on average. As a result, there is a decrease in erythrocytes (red blood cells), hemaglobin concentration, and blood viscosity (hematocrit). The blood is thinned in this way, which improves its flow properties and allows the placenta (placenta) to be optimally supplied with blood.
In addition to the increase in fluid in the plasma and erythrocytes, the increased amounts of fluid in the placenta, uterine muscle (uterine musculature), and fetus contribute to the weight gain of the pregnant woman. The content of the mother’s amniotic fluid and tissue also increases during pregnancy. Furthermore, there is an increase in the fat content of the mother of about 1.5 to 3.5 kilograms. These fat deposits are needed in particular as an energy reserve for breastfeeding. A weight gain during pregnancy of between 9 and 18 kilograms is considered normal due to the increase in total body water and fat deposits.