Pregnant women lack accurate knowledge of their BMI and recommended gestational weight gain

To investigate pregnant women's knowledge of their body mass index (BMI) and their knowledge of gestational weight gain guidelines.

Participants were recruited when attending their nuchal translucency scan at between 11 and 13 weeks, 6-days gestation in Dunedin or Christchurch, New Zealand. Recruitment staff measured participants' weight and height. By way of a self-administered, paper-based survey, participants were asked to identify their body size (including: underweight (BMI <18.5 kg/m2); normal weight (18.5-24.9); overweight (25-29.9); and obese (≥30)), and recommended gestational weight gain (including the 2009 Institute of Medicine guidelines for healthy weight gain in pregnancy, along with the options: "I should not gain any weight in my pregnancy", plus "It does not matter how much weight I gain"). Participant-measured BMI was compared to responses for perceived BMI and recommended gestational weight gain to assess accuracy. Demographic predictors of accuracy were also investigated.

In total, 644 women were included. Sixty-six percent of these correctly identified their BMI category, however only 31% identified their correct gestational weight gain recommendation. Overweight and obese women were much more likely to underestimate their BMI than normal weight women (p<0.001 for both). Overweight and obese women were also more likely to overestimate their weight gain recommendation (OR=4, p<0.001; OR=18, p<0.001, respectively) while normal weight women were more likely to underestimate their weight gain recommendation (p<0.001). Independent of BMI, women of New Zealand European ethnicity were less likely to underestimate their recommended gestational weight gain compared to other women of non-Māori/non-Pacific Island ethnicity (p=0.001), whereas younger women (p=0.012) were more likely to underestimate recommended gestational weight gain.

The present study indicates that New Zealand women, particularly those who are overweight and obese, lack accurate knowledge of their own body size, and this may lead to an under- or over-estimation of appropriate gestational weight gain, which may in turn lead to increased risk of poor health outcomes in pregnancy. Education strategies related to healthy weight gain in pregnancy are urgently required.