Expecting mother’s body goes through different, and often challenging developments throughout pregnancy. These physiological changes prepare the body to host and support both the intra-uterine and post-partum growth and development of the baby.

In this post, we intend to review Gestational Weight Gain (GWG), its relationship to a healthy pregnancy and the major determining factors related to the recommended or expected rates of GWG.

Body Mass Index (BMI)

To begin with, several indices are used by healthcare professionals to assess the adiposity and physical composition of individuals. The Body Mass Index (BMI), which is calculated as weight (kilograms) / height ² (meters), is the most frequently used. This allows for the classification of physical fitness and planning for optimal weight management and risk reduction.

It is worth noting that BMI should be carefully applied with consideration of Ethnic and socio-economic differences between Individuals to prevent harmful and negligible stigmatization.

In addition, factors such as genetics, biological, socioeconomic, political, environmental, and other determinants of body composition, along with individual risk factors should be considered when assessing and planning for proper weight management through diet and exercise.

Other indicators of weight management used by practitioners, dietitians, and nutritionists include waist circumference, waist-to-hip ratio, and specific body weight.

Mental health issues such as Bulimia Nervosa or Anorexia Nervosa closely correlate with body weight and BMI and could significantly affect both the mother and the baby. Such conditions need to be addressed and properly controlled under the care of medical physicians, preferably before the start of pregnancy.

Weight Classes (WHO)

Obesity and underweight classes are defined based on BMI by the World Health Organization (WHO). They are used not to stigmatize and label individuals but as a means of practical implementation for health and weight management recommendations.

Based on BMI classification by WHO, obesity classes I to III (BMI 30-34.9, 35 – 39.9, > 40, respectively) as well as mild, moderate, and severe underweight classes (BMI 17 – 18.49, 16 – 16.99, and < 16, respectively) are defined.

BMI in pregnancy

For prenatal care purposes, the pre-pregnancy BMI is a major determinant in the goals of weight gain through pregnancy.

According to the Association of Ontario Midwives (AOM) Clinical Practice Guidelines, a pre-pregnancy BMI between 18.5 and 24.9 is recommended for the optimal childbearing journey, as a BMI less than 18.5 is considered underweight, a BMI between 25 – 29.9 is considered overweight and a BMI above 30 is considered as obese.

Besides the risk of developing complications among underweight and overweight/obese women, as well as their weight gain goals during pregnancy vary, depending on their weight classes. Gestational Weight Gain within the recommended range is associated with better outcomes in pregnancy.

For example, a total weight gain between about 11 – 16 kg with a rate of 0.35 – 0.45 kg per week in the second and third trimesters is expected for those in the recommended BMI range (18.5 – 24.9 kg/m²)

Table 1 outlines the BMI classifications and expected gestational weight gain based on Institute of Medicine recommendations, adopted from the Association of Ontario Midwives Clinical Practice Guidelines.

Conclusion

To summarize, pre-pregnancy BMI plays a determining role in risk management throughout pregnancy, with overweight or underweight individuals facing an increased risk of complications. Pre-pregnancy BMI also correlates with proper gestational weight gain, as a higher-, or lower-than-expected GWG could put the pregnancy at increased risk of complications.

While it is beneficial to stay informed and educated on the subject and monitor your weight changes throughout pregnancy in a close relationship with your healthcare provider, one might not need to make drastic changes to their lifestyle and postpone their plans for pregnancy only considering and obsessing over their BMI score, as many factors affect an individual’s body composition and BMI.

It is recommended to keep a diverse and healthy diet and an active lifestyle following your individual, cultural, social, and economic preferences and track your pregnancy’s progress under the observation of a healthcare professional.

Table 1 – Institute of Medicine recommendations for total and rate of gestational weight gain by pre-pregnancy BMI
Pre-pregnancy BMI	BMI (WHO) (kg/m2)	Total gestational weight gain (kg)	Rate of gestational weight gain in second and third trimester (kg per week)
Underweight	< 18.5	12.5 – 18	0.45 – 0.6
Recommended	18.5 – 24.9	11 – 16	0.35 – 0.45
Overweight	25 – 29.9	6.5 – 11	0.2 – 0.3
Obese	≥ 30	5 – 9	0.18 – 0.28