Third trimester fatigue is real, it’s intense, and it has clear biological reasons. Your body is burning roughly 390 extra calories per day just to maintain the pregnancy by this stage, and more than half of that increased energy demand has nothing to do with the weight you’ve gained. It comes from your heart working harder, your lungs working harder, and the surprisingly high metabolic rate of the baby itself. Add disrupted sleep, possible anemia, and the physical strain of carrying significant extra weight, and exhaustion in these final months is nearly universal.
Your Body Is Running a Marathon at Rest
The most fundamental reason you’re so tired is that your metabolism has shifted dramatically. By the third trimester, your total daily energy expenditure increases by about 420 calories compared to early pregnancy. To put that in perspective, your baby’s tissues burn energy at roughly four times the rate of adult tissue per kilogram of body weight. Your body is fueling that around the clock.
Your cardiovascular system is also working overtime. Blood volume increases by 30 to 50 percent during pregnancy, peaking around 34 weeks. To move all that extra blood, your heart increases its output by about 40 percent, mostly by pumping more blood per beat but also by raising your resting heart rate by 10 to 20 beats per minute. Your heart is literally beating faster and harder than it was before pregnancy, even when you’re sitting on the couch. That costs energy, and it makes physical activity feel far more taxing than it normally would.
Sleep Gets Worse as Pregnancy Progresses
Fatigue scores in studies climb steadily from the first trimester through the third, and sleep quality follows the same downward trend. A Canadian meta-analysis confirmed what most pregnant people already know: sleep deteriorates as pregnancy progresses. Several specific culprits are responsible.
Nighttime urination is one of the biggest disruptors. Your kidneys are filtering a much larger blood volume, producing more urine overall, and the enlarged uterus presses on your bladder, reducing its capacity. The result is multiple wake-ups per night that fragment your sleep cycles and prevent the deep, restorative phases of rest your body needs most.
Physical discomfort also plays a major role. The ligaments in your pelvis loosen thanks to the hormone relaxin, the symphysis pubis widens, and your hips shift outward. These changes, combined with the weight of the baby, can make it difficult to find any sleeping position that doesn’t cause hip, back, or pelvic pain. Leg cramps are common and tend to strike at night.
Restless legs syndrome affects up to 27 percent of pregnant women, a rate much higher than in the general population. It creates an uncomfortable urge to move your legs that’s worst when you’re lying down trying to fall asleep. One small study found that folate supplementation dramatically reduced the incidence, though more research is needed to confirm that finding broadly.
Anemia Is Extremely Common
Iron deficiency anemia is one of the most overlooked causes of severe third trimester fatigue. Your body needs significantly more iron to support the 30 to 50 percent increase in blood volume, and many women can’t keep up through diet alone. The World Health Organization defines anemia in pregnancy as hemoglobin below 11 g/dL, and studies have found that more than 70 percent of women in the third trimester fall below that threshold, with the majority experiencing moderate anemia.
When you’re anemic, your blood carries less oxygen to your muscles and organs. The result is a bone-deep tiredness that rest alone won’t fix. If your fatigue feels disproportionate, or if it came on suddenly rather than building gradually, it’s worth asking your provider to check your iron levels. This is a treatable cause of exhaustion, not just something to push through.
Blood Sugar Fluctuations and Gestational Diabetes
Pregnancy hormones change the way your body processes sugar. Insulin, which normally moves glucose from your blood into your cells for energy, becomes less effective in the second and third trimesters. For most women, the pancreas compensates by producing more insulin. But when it can’t keep up, blood sugar rises and gestational diabetes develops.
Tiredness is one of the key symptoms of gestational diabetes, alongside frequent urination, excessive thirst, and nausea. The fatigue happens because glucose isn’t reaching your cells efficiently, so your body is energy-starved even when you’re eating enough. Most providers screen for gestational diabetes between 24 and 28 weeks, but if your exhaustion is accompanied by unusual thirst or more frequent urination than expected even for late pregnancy, mention it at your next appointment.
Stress and Anxiety Drain Energy Too
The psychological weight of late pregnancy is a real contributor to physical fatigue. Anxiety about labor, the health of the baby, financial concerns, and the life changes ahead can all activate your stress response. Research has found a significant relationship between sleep quality and perceived stress in pregnant women, creating a cycle where worry disrupts sleep and poor sleep amplifies worry.
This isn’t separate from the physical fatigue. It compounds it. When your nervous system is running in a heightened state, it burns energy and makes it harder to fall asleep or stay asleep. The mental load of preparing for a newborn, including the “nesting” urge to organize and clean, can also push you to overextend physically during the hours you feel most capable, leaving you depleted afterward.
How to Manage the Exhaustion
You can’t eliminate third trimester fatigue entirely because much of it is driven by biological changes you can’t control. But you can reduce how severe it feels.
Eating enough is more important than many people realize. Your body needs roughly 400 extra calories per day in the third trimester, and those calories need to include adequate protein and iron-rich foods. Falling short on calories forces your body to run a caloric deficit on top of an already demanding metabolic load, which compounds the tiredness significantly. Smaller, more frequent meals can also help stabilize blood sugar throughout the day.
Sleep positioning matters both for safety and comfort. Current clinical guidelines recommend avoiding falling asleep on your back after 28 weeks, as evidence suggests this position roughly doubles the odds of stillbirth compared to side-sleeping. The good news: sleeping on either side is equally safe. You don’t need to stay on your left side specifically. Using pillows between your knees, behind your back, and under your belly can help you maintain a side position through the night and reduce hip and back pain. A pillow wedged behind you also makes it less likely you’ll roll onto your back while asleep.
Gentle movement during the day, even short walks, can improve sleep quality at night and reduce the severity of restless legs symptoms. It sounds counterintuitive when you’re exhausted, but mild activity tends to improve energy levels more than complete rest does, as long as you’re not pushing into real exertion.
When Fatigue Signals Something More Serious
Most third trimester fatigue is normal. But certain combinations of symptoms warrant immediate attention. Preeclampsia, a serious pregnancy complication involving high blood pressure and organ stress, can cause fatigue alongside more distinctive warning signs: severe headaches, visual disturbances like blurriness or seeing spots, sudden swelling in your face or hands, severe upper abdominal pain, or sudden significant weight gain. Any of these alongside unusual exhaustion is a reason to contact your provider right away or go to an emergency room.
Fatigue that feels dramatically worse than it did even a week or two ago, breathlessness at rest, a racing heart that doesn’t settle down, or fainting episodes also fall outside the range of normal third trimester tiredness and deserve a medical evaluation. The most common explanation will still be something manageable like anemia, but ruling out more serious conditions matters.