Feeling profoundly exhausted as you approach the end of your pregnancy is a common experience. At 37 weeks, you are considered full term, and your body is operating under immense physiological demands that require massive energy. The fatigue is a direct result of these complex processes and is often a normal signal that your body requires deep rest. Understanding the specific physical changes, hormonal signals, and metabolic shifts occurring now can help explain why this intense tiredness sets in during the late third trimester.
The Physical Burden of the Third Trimester
The physical workload of late-stage pregnancy is a primary driver of exhaustion, creating energy expenditure even while resting. Your cardiovascular system is working overtime. Cardiac output is increased by up to 45% compared to before pregnancy, and blood volume has expanded by 40% to 50% to supply both your organs and the developing fetus. This sustained, elevated effort mimics the strain of continuous exercise.
The mechanical strain of carrying the baby’s weight also contributes to fatigue and musculoskeletal discomfort. Shifting posture and the relaxation of ligaments strain the back and hips, requiring constant muscular adjustments to maintain balance. The growing uterus compresses the diaphragm, which reduces lung capacity and leads to mild breathlessness (dyspnea). This reduction in respiratory efficiency means your body must work harder for oxygen exchange, further draining energy reserves.
Your resting metabolic rate (RMR) is substantially higher in the third trimester as the body functions as a 24/7 production line. This increase in RMR is estimated to rise by about 390 calories per day, representing the energy required to sustain the growing fetus, the placenta, and maternal tissues. This heightened metabolism means your body is burning energy at an accelerated rate even when resting, making true restorative rest difficult to achieve.
Hormonal Shifts and Sleep Interference
Internal chemical signals and physical factors disrupt the sleep patterns necessary for energy restoration. The hormone progesterone, produced by the placenta, contributes to fatigue. While it promotes the smooth muscle relaxation necessary to maintain the pregnancy, this effect also slows digestion and can exacerbate issues like heartburn and acid reflux, which frequently interrupt sleep.
Sleep is often fragmented at 37 weeks due to physical discomforts, preventing restorative sleep stages. Frequent nocturnal urination, driven by the pressure of the baby on the bladder, forces multiple awakenings each night. Physical discomforts like backache, restless legs syndrome, and mild Braxton Hicks contractions make finding a comfortable sleeping position challenging.
Mental fatigue is another factor, as anxiety and emotional preparation for labor and parenthood take a toll on cognitive energy. Hormonal surges, including oxytocin, can contribute to a sense of “nesting” energy followed by profound exhaustion.
Nutritional Needs and Energy Reserves
Internal resource depletion can compound the physical and sleep-related causes of fatigue. The most common nutritional factor contributing to late-pregnancy exhaustion is iron deficiency anemia, which affects nearly 30% of women in the third trimester. Plasma volume expands more rapidly than the red blood cell mass, leading to a “dilutional” effect that lowers hemoglobin concentration. Since iron is needed to produce the hemoglobin that carries oxygen, a deficiency directly reduces the blood’s oxygen-carrying capacity, resulting in tiredness and weakness.
The high metabolic demands place continuous stress on blood sugar regulation, which can cause energy dips even without diagnosed gestational diabetes. The developing fetus draws a constant supply of glucose from the mother, requiring a steady energy intake to prevent lethargy. Consuming small, frequent meals containing complex carbohydrates and protein can help maintain stable blood sugar levels throughout the day.
Dehydration can exacerbate feelings of lethargy and exhaustion. Increased blood volume and the body’s need to process fluids for both mother and baby require consistent water intake. Drinking water consistently throughout the day helps maintain optimal hydration and can improve overall energy levels.
When to Consult Your Healthcare Provider
While profound fatigue is often a normal part of the final weeks of pregnancy, certain accompanying symptoms warrant immediate medical attention. Fatigue that prevents basic daily function or is suddenly accompanied by other signs could indicate a more concerning medical issue.
You should contact your healthcare provider immediately if your fatigue is paired with symptoms of preeclampsia, a serious condition involving high blood pressure that can arise after 20 weeks.
Warning Signs Requiring Evaluation
If your fatigue is accompanied by any of the following, seek medical attention:
- A persistent, severe headache that does not improve with pain relief.
- Sudden, severe swelling in the face or hands.
- Changes in vision such as blurring or seeing spots.
- Pain in the upper right side of the abdomen, just below the ribs.
- Extreme paleness, dizziness, or shortness of breath that is significantly worse than usual (potential severe anemia).
- Signs of infection, such as fever or chills.
- Overwhelming feelings of sadness, inability to cope, or emotional distress that significantly interferes with your life.