Nausea is a symptom most people associate with the first trimester of pregnancy, often referred to as morning sickness. However, for a notable portion of pregnant individuals, feelings of sickness can reappear or worsen during the final three months. This recurrent third-trimester nausea is typically not caused by the same hormonal surges as early pregnancy sickness. Instead, it results from distinct physiological changes, primarily related to the physical constraints of the growing fetus and ongoing systemic adjustments.
Physical Compression of the Stomach and Esophagus
The most immediate and direct cause of late-stage digestive discomfort is the sheer size of the growing uterus. As the fetus reaches its maximum size in the third trimester, the uterus expands upward, displacing the abdominal organs. This mechanical pressure significantly reduces the available space for the stomach and intestines.
The upward pressure on the stomach directly impacts its function, often leading to delayed gastric emptying. Food remains in the upper digestive tract for extended periods, contributing to feelings of fullness, bloating, and ultimately, nausea.
This compression also affects the lower esophageal sphincter, the muscle ring that separates the stomach from the esophagus. The increased intra-abdominal pressure pushes stomach contents and acid back up, a condition known as gastroesophageal reflux (GERD) or heartburn. While heartburn is often characterized by a burning sensation, the irritation of the esophagus caused by reflux can manifest as a feeling of generalized sickness or nausea.
The diaphragm, the primary muscle for breathing, is also physically displaced upward by the expanding uterus. This displacement can increase pressure on the stomach even further. Managing meal size becomes important in the third trimester, as a full stomach exacerbates the mechanical forces contributing to reflux-induced nausea.
Late-Stage Hormonal and Systemic Factors
While the initial nausea of the first trimester is heavily linked to the rapid rise of human chorionic gonadotropin (hCG), late-stage nausea is influenced by other persistent hormonal effects. Progesterone, a hormone that remains at high levels throughout pregnancy, causes a widespread relaxation of smooth muscle tissue. This relaxation extends to the entire digestive tract.
The effect of progesterone significantly slows down the movement of food through the intestines, a process called reduced gut motility. This slower transit time, particularly in the small bowel, contributes to overall digestive sluggishness and discomfort that can be perceived as nausea.
Systemic changes in the third trimester contribute to sickness. Blood sugar fluctuations may occur, leading to feelings of lightheadedness or queasiness that mimic nausea. Additionally, heightened olfactory sensitivity may persist, making strong smells from food or the environment continue to be triggers for nausea.
The sheer physical exhaustion and discomfort that accumulate in the final weeks of pregnancy are also contributing factors. Difficulty sleeping, increased body weight, and general fatigue can lower the threshold for feeling unwell. This systemic stress often compounds the mechanical and hormonal issues, making a person more susceptible to feeling nauseated.
When Nausea Requires Medical Attention
Although mild, recurrent third-trimester nausea is common, a sudden onset or significant worsening of nausea and vomiting can signal a more serious underlying condition. It is important to distinguish typical discomfort from symptoms that require immediate medical evaluation. Vomiting so severe that it prevents the retention of liquids is concerning, as it can quickly lead to dehydration.
Signs of severe dehydration include persistent thirst, dark urine, and reduced frequency of urination. New or persistent nausea can also be a symptom of preeclampsia, a serious condition characterized by high blood pressure and signs of damage to organ systems, most often the liver and kidneys. Preeclampsia usually develops after 20 weeks of pregnancy.
Other warning signs associated with preeclampsia that may accompany nausea include:
- A severe headache that does not improve with standard pain relief.
- Visual changes such as blurred vision or seeing flashing lights.
- Sudden, significant swelling in the face or hands.
- Pain in the upper right side of the abdomen, just below the ribs, which can indicate liver involvement.
Anyone experiencing these red flags should contact their healthcare provider immediately for a full evaluation.