Pregnancy is a time of rapid physiological change, and the second trimester (weeks 13‑27) marks the period when the fetus’s growth accelerates most dramatically. Because the developing baby, placenta, and maternal tissues all require additional energy, most health authorities advise that pregnant people increase their daily caloric intake during this stage. Understanding how many extra calories are needed—and why the recommendation is not a one‑size‑fits‑all figure—helps expectant parents make informed decisions about nutrition, weight management, and overall health throughout mid‑pregnancy.
Typical Caloric Increase Recommended in the Second Trimester
Most national and international nutrition guidelines converge on a modest, yet measurable, increase in daily energy intake during weeks 13‑27. The most frequently cited figures are:
| Source | Recommended Extra Calories per Day (Second Trimester) |
|---|---|
| Institute of Medicine (IOM, 2009) | ≈ 340 kcal |
| World Health Organization (WHO, 2004) | ≈ 300 kcal |
| Academy of Nutrition and Dietetics (AND, 2022) | 300‑350 kcal |
| National Health Service (NHS, UK, 2021) | ≈ 300 kcal |
These numbers represent the average increase above a non‑pregnant baseline. They are intended for a pregnant person with a pre‑pregnancy body mass index (BMI) in the normal range (18.5‑24.9 kg/m²) and a sedentary to lightly active lifestyle. The figure is deliberately conservative; it supplies enough energy to support fetal growth and maternal tissue expansion without encouraging excessive weight gain.
Factors That Influence Individual Caloric Needs
While the baseline recommendation hovers around 300‑340 kcal per day, several personal and physiological variables can shift the required amount upward or downward:
| Factor | How It Alters Caloric Needs |
|---|---|
| Pre‑pregnancy BMI | Underweight individuals may need a larger surplus (≈ 350‑400 kcal), whereas overweight/obese individuals often require a smaller increase (≈ 200‑250 kcal). |
| Maternal age | Older pregnant people (≥ 35 y) may experience a slightly higher basal metabolic rate, modestly raising the extra calorie requirement. |
| Ethnicity and genetic background | Certain populations have different average basal metabolic rates and body composition patterns, influencing the net energy surplus needed. |
| Gestational weight gain goals | Clinicians set individualized weight‑gain targets based on BMI; the caloric surplus is adjusted to meet those targets. |
| Metabolic health | Conditions such as thyroid disorders or insulin resistance can affect resting energy expenditure, altering the extra calories needed. |
| Lifestyle and occupation | Physically demanding jobs or high‑intensity hobbies increase total energy expenditure, potentially necessitating a larger caloric addition. |
These determinants are why health professionals emphasize personalized nutrition counseling rather than a blanket prescription.
How Maternal Body Composition Affects Calorie Requirements
During the second trimester, the maternal body undergoes several compositional changes:
- Expansion of Blood Volume – Approximately 1.5 L of additional plasma is produced, raising the metabolic cost of circulating and maintaining this fluid.
- Growth of Uterine and Breast Tissue – Both tissues proliferate, requiring protein synthesis and cellular turnover, which consume energy.
- Accumulation of Maternal Fat Stores – The body preferentially stores extra energy as adipose tissue, especially in the abdominal and gluteofemoral regions, to serve as a reserve for the third trimester and lactation.
The energy cost of building new tissue is roughly 5‑7 kcal per gram of protein and 9 kcal per gram of fat deposited. Consequently, a pregnant person who gains more lean tissue (e.g., due to higher baseline muscle mass) may need a slightly larger caloric surplus than someone whose weight gain is predominantly adipose.
Impact of Fetal and Placental Growth on Energy Demands
The fetus’s weight roughly triples between weeks 13 and 27, reaching about 500 g by the end of the second trimester. Simultaneously, the placenta—an organ with a high metabolic rate—grows to a mass of 500‑600 g. The combined energy cost of fetal and placental development can be broken down as follows:
| Component | Approximate Energy Requirement (per day) |
|---|---|
| Fetal tissue accretion | 100‑120 kcal |
| Placental tissue accretion | 30‑40 kcal |
| Amniotic fluid production | 15‑20 kcal |
| Increased basal metabolic rate (BMR) due to hormonal shifts | 80‑100 kcal |
| Total | ≈ 300‑350 kcal |
These figures illustrate why the recommended extra calories align closely with the physiological cost of building new fetal and placental mass, plus the modest rise in maternal BMR driven by progesterone, estrogen, and thyroid hormone changes.
Variations for Different Pre‑Pregnancy BMI Categories
Guidelines from the IOM and other bodies stratify recommended gestational weight‑gain ranges by pre‑pregnancy BMI. The associated caloric surplus is derived from the target weight‑gain velocity (grams per week) for each BMI class:
| Pre‑Pregnancy BMI | Target Weight‑Gain Rate (2nd Trimester) | Approx. Extra Calories Needed |
|---|---|---|
| Underweight (< 18.5) | 0.44 kg/week | 350‑400 kcal |
| Normal (18.5‑24.9) | 0.35 kg/week | 300‑340 kcal |
| Overweight (25‑29.9) | 0.23 kg/week | 200‑250 kcal |
| Obese (≥ 30) | 0.18 kg/week | 150‑200 kcal |
The caloric numbers are derived from the energy density of weight gain (≈ 7 kcal per gram of tissue). For example, a normal‑weight individual gaining 0.35 kg per week (≈ 350 g) would need roughly 2,450 kcal extra per week, or about 350 kcal per day, above baseline needs.
Considerations for Multiple Pregnancies
Twin or higher‑order gestations substantially increase the metabolic load. The second trimester caloric surplus for twins is typically ≈ 600‑650 kcal per day above baseline, reflecting the need to support two fetuses, a larger placenta, and greater maternal tissue expansion. However, the exact figure depends on the same BMI‑based adjustments described above, and clinicians often monitor weight gain more closely in multiple pregnancies to avoid both under‑ and over‑nutrition.
Potential Consequences of Inadequate or Excessive Calorie Intake
Understanding the energy balance is not merely academic; it has tangible health implications:
- Insufficient Caloric Intake
- May limit fetal growth, increasing the risk of low birth weight or intrauterine growth restriction (IUGR).
- Can exacerbate maternal fatigue, impair immune function, and heighten the likelihood of nutrient deficiencies.
- May trigger excessive mobilization of maternal fat stores, potentially leading to ketosis in severe cases.
- Excessive Caloric Intake
- Accelerates gestational weight gain beyond recommended ranges, raising the odds of gestational diabetes mellitus (GDM), hypertensive disorders, and cesarean delivery.
- Increases the probability of postpartum weight retention and long‑term obesity for the mother.
- May predispose the child to higher birth weight, which is associated with later childhood obesity and metabolic syndrome.
Balancing the extra calories with nutrient density (adequate protein, essential fatty acids, vitamins, and minerals) is essential, but the focus here remains on the quantitative energy requirement.
Key Takeaways
- The average extra caloric need in the second trimester is ≈ 300‑340 kcal per day for a pregnant person with a normal pre‑pregnancy BMI and a sedentary to lightly active lifestyle.
- Individual factors—including BMI, age, metabolic health, activity level, and pregnancy type (singleton vs. multiple)—modify this baseline figure.
- The surplus primarily fuels fetal and placental tissue growth, maternal blood‑volume expansion, and a modest rise in basal metabolic rate.
- Under‑ or over‑consumption of these additional calories can have measurable adverse outcomes for both mother and baby, underscoring the importance of personalized nutrition guidance.
By recognizing that the second trimester’s caloric increase is a physiologically grounded, modest adjustment, expectant parents can approach nutrition with confidence, ensuring that the energy supplied aligns with the body’s genuine demands during this pivotal stage of pregnancy.
