Pregnancy significantly alters cardiovascular health, influencing both pregnancy outcomes and long-term postpartum health. Gestational diabetes and hypertensive disorders during pregnancy are associated with adverse outcomes such as pre-eclampsia, cesarean section, perinatal mortality, and increased risk of type 2 diabetes postpartum. Clustering of cardiovascular risk factors (glycemia, blood pressure, triglycerides, HDL cholesterol) within metabolic syndrome (MetS) is strongly linked to adverse pregnancy outcomes. While obesity is a major risk factor, BMI—a relatively poor marker of body fatness—cannot differentiate between fat mass (FM) and fat-free mass (FFM), which may have different health impacts. Few studies have examined the relationship between accurate body composition measurements and cardiovascular health in pregnancy. Additionally, the role of cardiorespiratory fitness in pregnant women's cardiovascular health remains largely unexplored. This study aimed to investigate the associations between body composition, physical fitness, and cardiovascular health (glycemia, gestational diabetes, insulin resistance, blood pressure, and MetS score) in early pregnancy.

Existing literature highlights the strong link between obesity and gestational diabetes, as well as other cardiovascular risks such as gestational hypertension and preeclampsia. However, BMI, commonly used to assess obesity, is an inadequate measure of body fatness in pregnancy. Accurate body composition assessment, differentiating FM and FFM, is needed to understand their distinct contributions to health outcomes. Studies using accurate methodologies have generally reported positive associations between FM and glycemia and insulin resistance during pregnancy, but the role of FFM remains unclear and has shown conflicting results in previous studies. Similarly, cardiorespiratory fitness is recognized as a crucial marker of health in non-pregnant individuals, potentially attenuating the negative effects of obesity, but its association with cardiovascular health in pregnant women is largely unknown.

This cross-sectional study used baseline data from the HealthyMoms trial (NCT03298555), a randomized controlled trial examining the effectiveness of a smartphone app for promoting healthy pregnancy weight gain, diet, and physical activity. The study included 303 women recruited in early pregnancy (around gestational week 14). Body composition (height, weight, body volume using air-displacement plethysmography) was measured, along with cardiorespiratory fitness (6-min walk test) and handgrip strength (dynamometer). Fasting blood samples were collected to analyze glucose, insulin, cholesterol, triglycerides, LDL, and HDL cholesterol. Gestational diabetes was defined as fasting plasma glucose ≥ 5.1 mmol/l. HOMA-IR was calculated, and blood pressure and MetS score (excluding waist circumference) were also determined. Linear and logistic regression analyses were performed to assess associations of body composition and fitness variables with cardiovascular health variables, adjusting for age, educational attainment, and parity. Sensitivity analyses were conducted to assess the robustness of the findings.

Body mass index (BMI), fat mass index (FMI), and % fat mass were strongly and positively associated with glucose, HOMA-IR, systolic and diastolic blood pressure, and MetS score. These associations remained robust even after adjusting for other variables. Fat-free mass index (FFMI) showed weaker and less consistent associations with cardiovascular health variables, and these associations were substantially attenuated after adjusting for FMI and physical fitness. Higher cardiorespiratory fitness was weakly associated with lower HOMA-IR and MetS score in unadjusted and partially adjusted models. However, these associations disappeared after adjusting for FMI and other variables. Handgrip strength showed no significant associations with any of the cardiovascular health variables. Odds ratios (ORs) revealed that a 1 SD increase in BMI, FMI, and % fat mass were significantly associated with higher odds of gestational diabetes, high HOMA-IR, high blood pressure, and high MetS score. The associations of FFMI with these outcomes were weaker and largely disappeared after adjustment for FMI. A 1 SD increase in cardiorespiratory fitness was associated with lower odds of high HOMA-IR and high MetS score in the unadjusted model, but this effect was eliminated after adjustment for FMI.

This study demonstrates strong associations between accurately measured fat mass (FMI and %FM) and multiple markers of cardiovascular health and gestational diabetes risk in early pregnancy. The strength of association with BMI was comparable to that of FMI and %FM, suggesting that BMI may be sufficient for initial risk assessment in clinical settings. The relatively weak and inconsistent associations observed with FFMI suggest that the primary focus in clinical care should be on managing excess fat mass rather than on fat-free mass. The finding that cardiorespiratory fitness is associated with reduced risk of high HOMA-IR and MetS score in early pregnancy is novel. However, these associations are likely mediated through the effects of fat mass, indicating that maintaining a healthy weight is crucial for cardiovascular health during pregnancy.

This study confirms the strong relationship between fat mass and adverse cardiovascular outcomes and gestational diabetes risk during pregnancy. While BMI serves as a reasonable initial indicator, focusing on reducing excess fat mass is crucial for improving cardiovascular health during and after pregnancy. The role of cardiorespiratory fitness requires further investigation, considering its potential mediation by fat mass.

The study’s cross-sectional design limits the establishment of causal relationships. The use of a submaximal measure of cardiorespiratory fitness (6-min walk test) might have influenced the results; future research should utilize maximal exercise testing. The sample's high proportion of women with high educational attainment might limit generalizability. The lack of waist circumference data in MetS calculation is a limitation. Finally, the inability to differentiate the body composition of the fetus from the mother might have introduced some degree of error.