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2012 research finding

Maternal weight and weight gain affect birth weight

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Pre-conception overweight among women and excessive weight gain during pregnancy are linked to an increased birth weight, according to a study from the Norwegian Mother and Child Cohort Study. Mothers with higher education and previous pregnancies also showed a correlation with increased birth weight, whereas lower birth weights were found with higher maternal age and smoking during pregnancy.

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  • baby ny.jpg
    The average birth weight was 3675 grams 
  • The average body mass index (BMI) before pregnancy was 24 kg/m2 
  • 22 per cent of pregnant women were overweight and 9.5 per cent were obese before conception 
  • Their weight increased on average 9.3 kg up to 30 weeks gestation 
  • The difference in birth weight between the lightest group (BMI <18.5) and heaviest (BMI> 40) was nearly 500 grams.

The 58,383 children were all born at term between 2000 and 2007, and twins and triplets were excluded from the study. Birth weight increased with maternal weight gain during pregnancy among underweight, normal weight and overweight mothers, but the effect of weight gain was greatest in the lightest mothers. The degree of obesity was also significant, with the group with the highest BMI having the highest birth weights.

The average birth weight among Norwegian children was highest around the turn of the millennium, and has fallen by about 50 grams over the last ten years. 4.7 per cent of Norwegian infants weighed over 4.5 kg in 2000, while the percentage in 2009 fell to 3.1 per cent. Obesity before and during pregnancy increases the risk of diabetes, pre-eclampsia, thrombosis and birth complications.

Other research suggests that higher birth weight may affect the child’s long-term regulation of appetite and obesity. Pediatrician Unni Stamnes Kopp and her research team will investigate this further in the Norwegian Mother and Child Cohort Study.


Stamnes Koepp, U. et al Maternal pre-pregnant body mass index, maternal weight change and offspring birthweight. Acta Obstetricia et Gynecologica Scandinavica, Vol 91, Issue 2, pg 243–249, Feb 2012