The experience of pregnancy involves a profound restructuring of the body’s interior architecture to accommodate a growing fetus. The abdominal cavity is an area of limited space, and the volume required for a full-term pregnancy necessitates a temporary displacement of the organs housed there. This dynamic process, driven by the expansion of the womb, explains many of the common physical sensations experienced throughout gestation.
The Engine of Change: Uterine Growth
The primary force behind all internal rearrangement is the uterus, an organ uniquely designed for extreme expansion. Before pregnancy, the uterus is a small, pear-shaped organ that is contained entirely within the pelvis. By the end of the first trimester, it has grown to roughly the size of a grapefruit and begins to rise out of the pelvic bowl. This initial growth starts the process of crowding neighboring structures like the bladder.
The rate of expansion accelerates dramatically during the second and third trimesters, pushing the uterus into the abdominal cavity. By the time of delivery, the uterus has transformed into a massive structure, reaching the size of a watermelon and extending from the pubic bone up to the bottom of the rib cage. The organ’s volume capacity increases significantly, which is the direct cause of the pressure and shifting within the abdomen.
Where Organs Go: The Stomach and Abdominal Cavity
As the uterus grows upward and forward, it acts like an expanding balloon, pushing the organs that normally occupy the lower and mid-abdomen out of the way. The stomach, which typically sits high in the abdominal cavity just beneath the diaphragm, is pushed further superiorly and compressed. Instead of maintaining its natural shape, the stomach is squeezed into a more vertical, elongated position, often pressing directly against the underside of the diaphragm and the rib cage.
The intestines are also significantly affected by this space constraint, being shunted laterally and superiorly around the perimeter of the expanding uterus. The sheer size of the full-term uterus, which reaches the xiphoid process (the bottom tip of the sternum), means there is minimal free space left in the abdominal area. This displacement is a compression, where the organs are forced to share a drastically reduced functional volume. Even the liver experiences a slight upward lift and compression against the diaphragm due to the pressure from below.
The Physical Effects of Organ Displacement
The physical repositioning and compression of the stomach and other organs lead directly to several well-known functional changes during pregnancy. One of the most frequent complaints is heartburn, or gastroesophageal reflux disease (GERD).
This occurs because the upward pressure from the uterus mechanically pushes the stomach contents toward the esophagus. Furthermore, pregnancy hormones cause the lower esophageal sphincter (LES), the valve separating the esophagus and stomach, to relax. This hormonal relaxation, combined with the mechanical upward squeeze of the stomach, allows stomach acid to more easily backflow, causing the characteristic burning sensation in the chest and throat.
The physical compression of the stomach also reduces its capacity, explaining why many pregnant people experience early satiety, or feeling full after eating only a small amount of food. Because the stomach is physically smaller, it reaches its limit much sooner than usual.
The upward displacement of the stomach and other organs also directly impacts the diaphragm, the large muscle responsible for breathing. As the uterus pushes up on the diaphragm, the muscle has less room to descend fully during inhalation, limiting the full expansion of the lungs. This restriction often results in the sensation of shortness of breath, or dyspnea, particularly in the third trimester. These symptoms typically resolve quickly after delivery when the uterus rapidly shrinks and the organs return to their original locations.