Pregnancy fatigue hits hard because your body is simultaneously building a new organ (the placenta), expanding your blood supply by nearly half, and running on hormones that act as natural sedatives. It’s not one thing making you tired. It’s a dozen biological changes happening at once, all competing for energy your body used to spend on you. Fatigue typically peaks around weeks six to eight of the first trimester, eases in the second trimester, then returns in the third as the physical demands of carrying extra weight take over.
Progesterone Acts Like a Sedative
The hormone progesterone surges early in pregnancy, and it’s one of the biggest reasons you feel like you could sleep 14 hours a day. Progesterone doesn’t just maintain the uterine lining for the embryo. It also crosses into the brain, where it gets converted into a compound called allopregnanolone, one of the most potent natural sedatives your body produces. The effect was first identified by a researcher who noticed that progesterone’s breakdown products could function as anesthetics. In early pregnancy, your progesterone levels climb rapidly, and that sedative effect comes along for the ride.
This is why first-trimester fatigue feels different from normal tiredness. It’s not just “I didn’t sleep well.” It’s a deep, heavy exhaustion that sleep doesn’t fully fix, because the hormone driving it is constantly circulating.
Your Heart Is Working Much Harder
During pregnancy, your total blood volume increases by roughly 45%, though it can range anywhere from 20% to 100% above your pre-pregnancy level. That’s a massive change. Your heart has to pump all that extra fluid, and your heart rate rises by 10 to 20 beats per minute over the course of pregnancy, a 20% to 25% increase over baseline. Even at rest, your cardiovascular system is working as though you’re doing light exercise.
This increased cardiac demand is one reason everyday activities feel more draining. Walking up stairs, carrying groceries, or standing for long periods all require more oxygen delivery than they did before. The American Heart Association notes that healthy pregnant women commonly experience increased shortness of breath on exertion and greater fatigue, even without any underlying heart condition.
Your Metabolism Burns More Calories
Growing a baby is metabolically expensive. Your basal metabolic rate, the number of calories your body burns just to keep itself alive, increases at an average rate of about 11 extra calories per gestational week. That may sound small, but it adds up. By the second trimester, your body needs roughly 350 extra calories per day compared to pre-pregnancy. By the third trimester, that number climbs to about 500 extra calories per day.
If you’re not eating enough to keep up with that increased demand, fatigue gets worse. But even when you are eating enough, your body is diverting a significant share of that energy toward building fetal tissue, expanding your blood supply, and maintaining the placenta. You’re running a higher-output engine all day, every day.
Blood Sugar Swings Start Early
The placenta produces a hormone called human placental lactogen, detectable in your blood as early as the sixth week. This hormone gradually pushes your body toward insulin resistance, meaning your cells don’t absorb blood sugar as efficiently as they normally would. The purpose is to keep glucose available for the growing baby, but the side effect for you is that your own energy supply becomes less stable.
By the second half of pregnancy, concentrations of this hormone increase tenfold. Your pancreas compensates by growing more insulin-producing cells and ramping up insulin output, but that compensation isn’t always seamless. The result can be energy dips, especially between meals, that feel like hitting a wall. This is also the mechanism behind gestational diabetes when the system can’t keep up with demand.
Iron Deficiency Is Extremely Common
Nearly 40% of pregnancies worldwide are complicated by anemia, and iron deficiency is the most common cause. In the United States alone, roughly 1 in 5 pregnant people develop iron-deficiency anemia. When your red blood cell count drops, your body can’t deliver oxygen to tissues efficiently, and the primary symptom is fatigue.
The reason iron deficiency is so prevalent during pregnancy is straightforward: your blood volume is expanding dramatically, and your body needs iron to make all those new red blood cells. If your iron stores were marginal before pregnancy, they can deplete quickly. Providers typically screen for anemia with a blood test, and a ferritin level below 30 can indicate iron deficiency even before your hemoglobin drops low enough to be classified as anemia. If your fatigue feels disproportionate or is getting worse rather than better in the second trimester, low iron is one of the first things worth checking.
Sleep Gets Worse, Not Better
Your body is demanding more rest at exactly the time it becomes harder to get quality sleep. Pregnancy disrupts sleep through multiple channels: hormonal shifts alter sleep architecture, the growing uterus presses on the bladder (leading to multiple nighttime bathroom trips), breathing patterns change, and discomfort from back pain or heartburn makes it harder to stay asleep. Even in the first trimester, when the uterus is still small, hormonal changes alone can fragment sleep enough to leave you unrested.
This creates a frustrating cycle. You’re biologically more fatigued than usual, but the sleep that should restore your energy is shallower, shorter, and more frequently interrupted. Many people describe first-trimester fatigue as feeling jet-lagged for weeks on end, and the comparison isn’t far off: your circadian rhythm and sleep quality are genuinely disrupted.
Why the Second Trimester Feels Better
Most people notice a real improvement in energy around weeks 13 to 14. The placenta is now fully formed and takes over hormone production, which stabilizes progesterone levels rather than continuing the sharp climb of the first trimester. Your body has also adapted to the increased blood volume, so your cardiovascular system isn’t working as hard relative to its new baseline. The nausea that often accompanies early pregnancy tends to ease, which means you’re eating and absorbing nutrients more effectively.
This second-trimester energy boost is real, but it’s relative. You likely won’t feel as energetic as you did before pregnancy. You’ll just feel noticeably better compared to the deep fatigue of the first trimester.
Why Fatigue Returns in the Third Trimester
The third trimester brings fatigue back for different reasons than the first. By now, you’re carrying significantly more weight, and your center of gravity has shifted forward due to the growing abdomen. Your body compensates by adjusting posture, which recruits muscles (particularly the calf and back muscles) that wouldn’t normally work as hard during simple standing or walking. That postural adjustment increases the energy cost of basic movement.
Sleep disruption also intensifies. The uterus is large enough to press heavily on the bladder, making nighttime urination more frequent. Finding a comfortable sleeping position becomes genuinely difficult. Meanwhile, your metabolic demands are at their peak, burning 500 extra calories a day. Your heart rate is at its pregnancy maximum. And if iron stores have been gradually depleting over the previous months, anemia may be contributing by this stage as well.
When Fatigue Signals Something Else
Normal pregnancy fatigue is persistent but manageable. You’re tired, but you can still get through your day. The American College of Obstetricians and Gynecologists flags “overwhelming tiredness” as a warning sign when it crosses certain lines: you’re suddenly much more tired and weak than your baseline, sleep doesn’t refresh you at all, or you can’t summon enough energy for daily activities.
That kind of fatigue can indicate anemia, thyroid dysfunction, diabetes, or depression. Postpartum, it can also be an early sign of heart problems. The distinction matters: normal pregnancy fatigue is gradual and tracks with the hormonal and physical changes described above. Fatigue that arrives suddenly, worsens sharply, or feels qualitatively different from what you’ve been experiencing is worth bringing up with your provider, even if it seems like “just being tired.”