Yes, metabolism increases significantly during pregnancy. Resting metabolic rate rises by roughly 20% from early to late pregnancy, with most of that increase happening in the second and third trimesters. This isn’t a subtle shift. By the end of pregnancy, your body is burning an extra 200 to 400 calories per day just to keep itself running at rest.
How Much Your Metabolism Rises by Trimester
The increase isn’t uniform across pregnancy. It follows a curve that accelerates as the demands on your body grow. A systematic review published in Nutrition Reviews found that resting energy expenditure increases between 0.5% and 18.3% from early to mid-pregnancy, then climbs further, reaching 6.4% to 29.6% above early-pregnancy levels by late pregnancy. That wide range reflects natural variation between individuals, but the upward trend is consistent.
One study tracking the same women across all three trimesters measured average resting energy expenditure at about 1,245 calories per day in the first trimester, 1,382 in the second (an 11% jump), and 1,524 in the third (a 22.4% increase over the first trimester). Research from the American Journal of Obstetrics and Gynecology found a similar pattern, with resting metabolic rate rising 19% on average from early to late pregnancy.
In practical terms, most people won’t notice much metabolic change during the first trimester. The real acceleration starts around mid-pregnancy and continues to build until delivery.
What’s Driving the Extra Energy Demand
Your body isn’t just growing a baby. It’s building and maintaining an entirely new organ (the placenta), expanding its blood supply, and ramping up the workload on several major organ systems simultaneously. Each of these draws energy.
The fetus itself is surprisingly metabolically active, demanding roughly 56 calories per kilogram of body weight per day in late pregnancy. But fetal growth is only part of the picture. Your heart, kidneys, and lungs are all working harder. Cardiac output, the volume of blood your heart pumps per minute, increases by 30% to 50% and peaks around 16 to 20 weeks of gestation. Your kidneys filter more blood, your lungs move more air, and all of that costs energy even when you’re sitting still.
There’s also the energy cost of building new tissue. Over the course of pregnancy, the body typically deposits around 0.9 kilograms of protein (in the form of uterine muscle, breast tissue, blood volume, and the baby itself) and about 3.8 kilograms of fat. Synthesizing and storing that tissue requires calories above and beyond what it takes to maintain it. One widely cited estimate puts the total energy cost of a full pregnancy at around 85,000 calories, or roughly 300 extra calories per day averaged across all nine months.
The Role of Hormones
Hormonal shifts begin reshaping your metabolism almost immediately after conception. In the first trimester, rising levels of hCG (the hormone detected by pregnancy tests) stimulate the thyroid gland, temporarily boosting levels of thyroid hormones that regulate how fast cells burn energy. This causes a brief dip in TSH, the hormone that normally tells the thyroid how much to produce. After the first trimester, thyroid hormone levels settle into a new pattern, but overall thyroid hormone production increases by about 50% to meet the demands of pregnancy.
Progesterone, which rises steadily throughout pregnancy, also plays a role. It raises your core body temperature slightly and contributes to increased respiratory drive, both of which add to baseline energy expenditure. These hormonal changes explain why some people feel warmer than usual or notice a faster resting heart rate well before any visible belly growth.
How Body Composition Plays In
Metabolic rate is closely tied to how much metabolically active tissue you carry. During pregnancy, your lean mass increases as your blood volume expands, your uterus grows, and the fetus develops. All of this tissue burns calories around the clock. Fat mass also increases, particularly during the second trimester, when the body is actively storing energy reserves for late pregnancy and breastfeeding.
Interestingly, the metabolic increase in pregnancy isn’t fully explained by the gain in lean tissue alone. Research shows that even after accounting for changes in fat-free mass, resting metabolic rate still rises more than expected. This suggests that pregnancy triggers metabolic changes at the cellular level, not just from carrying more tissue. Your existing organs are genuinely working harder.
By the third trimester, the body shifts strategy. Fat storage slows down considerably as more energy is redirected toward the rapidly growing fetus, which roughly triples in weight during the final 10 weeks.
What This Means for Calorie Needs
A higher metabolism means your body needs more fuel, but the timing matters. In the first trimester, energy requirements are essentially the same as before pregnancy. The recommended increase is about 340 additional calories per day in the second trimester and 452 in the third. These numbers align with the metabolic data: your body doesn’t need much extra early on, but demand ramps up substantially in the back half of pregnancy.
These are averages, though. Actual calorie needs vary depending on your pre-pregnancy weight, age, and activity level. Someone who was very active before pregnancy and continues exercising will have higher total energy needs than someone who is more sedentary. The metabolic increase from pregnancy stacks on top of whatever your baseline was, so there’s no single number that applies to everyone.
The rise in metabolism also helps explain some of the physical sensations of pregnancy: feeling warmer, sweating more easily, getting hungry more often, and feeling fatigued even without extra physical activity. Your body is doing more work at rest than it used to, and that has real, noticeable effects on how you feel day to day.