Metabolism is the process by which the body converts food into energy. During pregnancy, the body’s energy needs change dramatically to support the developing fetus and the physiological adaptations of the birthing person. This requires a substantial increase in the rate at which energy is produced and consumed, meaning that metabolism significantly increases throughout pregnancy. This adjustment ensures that both the developing fetus and the birthing parent remain nourished.
Confirming the Metabolic Shift
The clearest confirmation of this change comes from measuring the energy the body uses at rest, known as the Basal Metabolic Rate (BMR), and the overall energy used throughout the day, the Total Energy Expenditure (TEE). The BMR begins to rise early in gestation, with a more pronounced increase typically observed from the second trimester onward. By the third trimester, the BMR may increase by as much as 25% over pre-pregnancy levels, reflecting the substantial increase in metabolically active tissue.
The rise in BMR accounts for a significant portion of the total additional energy cost of pregnancy. This increase is cumulative, averaging about 5% in the first trimester, 10% in the second, and peaking at roughly 25% in the third trimester. The Total Energy Expenditure also increases, though the magnitude can vary widely among individuals, ranging from approximately 8% to 33% by late pregnancy.
This variation in TEE is partly due to changes in physical activity, as some individuals naturally reduce their activity levels as the pregnancy advances. Even with reduced activity, the overall energy requirement remains higher due to the elevated BMR. The increase in metabolic rate is a normal physiological adaptation to the demands of the pregnancy.
Energy Demands of Fetal and Maternal Tissue Growth
The reason for the increased metabolic rate is the intense biological work required to build and maintain the pregnancy. The developing fetus is a major energy consumer, especially during the later stages of gestation when growth is most rapid. Fetal growth requires a continuous supply of energy and nutrients to build new tissues and organs.
Beyond the fetus, several maternal tissues and organs experience significant growth and increased function, contributing to the elevated energy demand. The uterus dramatically increases in size, the mammary glands prepare for lactation, and blood volume expands by 40% to 50%. All of this tissue growth requires energy for synthesis and maintenance.
The placenta is another organ with a high metabolic rate, functioning as the interface for nutrient and waste exchange. This highly active organ consumes energy to perform its regulatory and transport functions. Furthermore, maternal support systems, such as the cardiac, respiratory, and renal systems, work harder to process the increased blood volume and metabolic waste, leading to a greater energy expenditure.
Adjusting Caloric Intake and Nutrient Handling
The increased metabolic demand translates directly into a need for additional caloric intake, though the concept of “eating for two” is largely a misconception. The total energy cost of a full-term pregnancy is estimated to be around 80,000 kilocalories, which, when averaged across the duration, suggests an increase of about 300 extra calories per day.
Caloric needs are not distributed evenly across the trimesters. Energy requirements are generally similar to pre-pregnancy levels during the first trimester, with recommendations for an additional 340 calories per day in the second trimester and about 450 calories per day in the third. These extra calories should come from nutrient-dense foods to support the specific needs of tissue growth, such as increased protein intake.
To ensure the fetus receives adequate fuel, the body adjusts its handling of macronutrients, particularly glucose and fat. In the later stages of pregnancy, a state of physiological insulin resistance develops, which reduces the maternal tissues’ ability to use glucose. This mechanism, driven by placental hormones like human placental lactogen, is a temporary adaptation that spares glucose and directs it toward the fetus.
The shift in fuel use also involves increased fat mobilization, which allows the birthing parent to use fat as a primary energy source, further conserving glucose for the fetus. The body’s management of these macronutrients is a sophisticated way of balancing the increased metabolic needs of both the birthing person and the growing fetus.