Gaining too little or too much weight in pregnancy affects birth outcomes

By Dr. Sanchari Sinha Dutta, Ph.D.Reviewed by Lauren HardakerNov 24 2025

An analysis covering 1.6 million pregnancies shows that both low and high weight gain can drive major complications, from preterm birth to C-sections. This raises urgent questions about how global guidelines should evolve for today’s diverse populations.

Study: Gestational weight gain and risk of adverse maternal and neonatal outcomes in observational data from 1.6 million women: systematic review and meta-analysis. Image credit: Natali Kuzina/Shutterstock.com

A systematic review and meta-analysis of observational data from 1.6 million women provide evidence on the association between weight gain during pregnancy (gestational weight gain) and risks of adverse outcomes in both mothers and newborns across diverse world regions and income settings. The findings are published in the British Medical Journal (BMJ).

Why gestational weight guidelines need updating

Gestational weight gain is known to be associated with acute and long-term health risks in both mothers and newborns. The Institute of Medicine (IOM) in the USA generated gestational weight gain guidelines in 1990 to optimize these outcomes, particularly low birth weight.

These guidelines were developed based on a moderate-sized population of mostly White US women with a mean body mass index (BMI) of 24 and an age of 26 years. In 2009, the guidelines were updated to incorporate World Health Organization (WHO) BMI categories and broader health outcomes. Since then, these guidelines have provided an important international reference point.

The populations that formed the basis of these guidelines were characterized by lower maternal age and BMI, with limited ethnic and socioeconomic diversity. Substantial variations in global population characteristics highlight the need for developing region-specific guidelines on gestational weight gain.

Global population trends over the past two decades have indicated an increase in maternal age, BMI, and gestational weight gain, further highlighting the urgent need for updated guidelines based on contemporary populations across diverse world regions and income settings.

The current systematic review and meta-analysis aimed to inform the WHO’s initiative to develop global standards for gestational weight gain.

A total of 40 observational studies with 1.6 million female participants aged over 18 years were analyzed to report maternal and infant outcomes stratified by participants’ BMI and gestational weight gain.

The study also evaluated additional outcomes such as gestational diabetes and neonatal jaundice, although these were less consistently reported across included studies.

Weight gain patterns alter risks

This systematic review and meta-analysis included contemporary evidence (2009-2024) from 1.6 million women across five WHO world regions. About 53 % of these women had normal BMI before pregnancy, 6 % were underweight, 19 % were overweight, and 22 % were obese.

Only 32 % of the entire population had gestational weight gain within the recommended ranges, with 23 % gaining less and 45 % gaining more than recommended. 

The meta-analysis of selected studies that used WHO BMI categories revealed that gestational weight gain below the recommended range is associated with a lower risk of caesarean delivery, a large-for-gestational-age infant (an infant with body weight significantly higher than the average for their gestational age), and macrosomia (birth weight significantly higher than the average), but a higher risk of preterm birth, a small-for-gestational-age infant, low birth weight, and respiratory distress. These associations were generally consistent across BMI categories, although the strength of the effects varied.

Gestational weight gain above the recommended range, on the other hand, was associated with a higher risk of caesarean delivery, increased birth weight, hypertensive disorders of pregnancy, a large-for-gestational-age infant, macrosomia, and neonatal intensive care unit (NICU) admission, and a lower risk of preterm birth and a small-for-gestational-age infant.    

Notably, the findings revealed that gestational weight gain below the recommended range is associated with a higher risk of hypertensive disorders of pregnancy in Asian women. However, substantial heterogeneity across Asian studies, driven partly by different BMI classification systems, limits the certainty of these estimates.

Guidance for global care

This systematic review and meta-analysis provide valuable evidence on the relationship between gestational weight gain and adversities in mothers and infants. This evidence will help inform the WHO's initiation process to develop globally relevant guidelines for gestational weight gain, aiming to improve post-pregnancy outcomes across world regions.

The meta-analysis included five Asian studies that used WHO BMI categories, and several other studies from China that used regional BMI categories, which limit the generalizability of the findings across Asia.

Overall, the findings from Asia indicate high levels of uncertainty, making it challenging to distinguish population-specific risks from BMI classification differences in this world region. Given that 60% of the global population resides in Asia, it is essential to understand how BMI category differences across regions influence variations in pregnancy outcomes due to gestational weight gain.

These Asian findings support the ongoing effort to incorporate diverse populations from different world regions and settings in developing globally relevant standards for gestational weight gain.

According to the meta-analysis findings, women belonging to the lowest obesity category (lowest BMI in the obese range) have a higher risk of having large-for-gestational-age infants compared to those belonging to other obesity categories. Although subgroup analyses by obesity class were performed, patterns across classes should be interpreted cautiously, as the study did not draw definitive comparative conclusions on this aspect.

Overall, the safety and implications of weight loss in women with a BMI in the obese range during pregnancy remain uncertain. Further research is needed across a broader range of outcomes, and with a standardized method for reporting BMI and gestational weight gain categories to reduce cross-study variability.

In conclusion, this review complements existing individual patient data by including a broad range of studies and outcomes, identifying key gaps to inform future individual patient data priorities within the WHO’s ongoing global gestational weight gain standards initiative.

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