Around two-thirds (68%) of pregnancies have weight gain that is more or less than recommended and that is associated with complications such as preterm birth, large birth weight, and admission to intensive care, finds a review of data from 1.6 million women published by The BMJ today.
These findings reinforce the need for international standards for healthy GWG alongside lifestyle support and public health measures to improve outcomes for mothers and babies worldwide, say lead researchers Helene Teede and Rebecca Godstein.
Gaining too much or too little weight during pregnancy (known as gestational weight gain or GWG - the combined growth of mother and baby) is associated with increased health risks for both mother and baby, which can continue into later life.
Most countries rely on Institute of Medicine GWG guidelines, but these are based on data from predominantly white women in high income countries in the 1980s so they don't reflect ethnically diverse populations across low, middle, and high income settings, or the increasing availability of unhealthy foods and other environmental factors that are driving a global trends such as rising weight (BMI).
In response, the World Health Organization (WHO) has launched an initiative to develop global healthy GWG standards aimed at defining optimal GWG recommendations across diverse settings.
To support this initiative, researchers analysed data from 40 observational studies involving 1.6 million women aged 18 and over from five of the six WHO defined world regions that reported pregnancy outcomes according to BMI and GWG from 2009 to 2024. Of these 40 studies, 36 (90%) were considered to be high quality.
Around half (53%) of study participants had a healthy pre-pregnancy BMI, with others classified as below (6%), above (19%), or well above healthy weight (obese) (22%). Only a third (32%) had GWG combined for mother and baby, within recommended ranges, with 23% gaining below and 45% gaining above recommended.
According to WHO BMI criteria, GWG below the recommended range was associated with lower risk of caesarean delivery, a large for gestational age infant, and high birth weight (macrosomia) but higher risk of preterm birth, a small for gestational age infant, low birth weight, and respiratory distress.
Conversely, GWG above the recommended range was associated with a higher birth weight and a higher risk of caesarean delivery, hypertensive disorders of pregnancy, a large for gestational age infant, macrosomia, and admission to a neonatal intensive care unit, and a lower risk of preterm birth and a small for gestational age infant.
Similar patterns were apparent when Asian BMI categories were used in studies conducted in this world region.
The researchers point to some limitations, such as variations in BMI and GWG classifications, and note that few studies from low income countries met their inclusion criteria, limiting diversity. Nor can they rule out the possibility that other unmeasured factors, such as smoking status, age, and ethnicity, may have influenced their results.
However, they conclude: "Our findings inform and support the need for optimised, evidence based WHO international GWG reference standards based on individual patient data, applicable across the full BMI range in contemporary and diverse global populations. This will build on and improve current recommendations and are essential to underpin multi level support to improve the health of mothers and babies worldwide."
Although the review largely confirms longstanding knowledge, its most striking insight is the considerable variation across categories of body mass index and regions, challenging the black and white logic that underpins many clinical guidelines, say Annick Bogaerts and Dominika Osicka in a linked editorial.
They suggest that rather than warning all women against exceeding fixed cut-offs, a more nuanced approach is needed, with a focus on patterns of weight gain, its underlying determinants, and personalised, non-punitive counselling.
"Without comprehensive, life course public health strategies, the obesity epidemic will continue across generations," they argue. "Governments and (inter)national agencies must act now to support women's health before, during, and after pregnancy, ensuring that the next generation inherits the opportunity not the risk."
