Wednesday June 2 2010
The study did not assess diet or activity in mothers and children
Women who “pile on the pounds” during pregnancy could be putting their child at risk of heart disease in later life, reported the Daily Mail.
This study assessed the relationship between mothers’ weight gain in pregnancy and weight before pregnancy, and their children’s body fat and heart (cardiovascular) disease risk. The results appear to show a positive association between greater than recommended weight gain during pregnancy and the child having more body fat at the age of nine.
However, many factors affect weight gain. Although the researchers considered many of these in their analysis, they did not include them all, such as diet and activity levels in mother and child. In addition, mother and child could potentially share genetic factors that affect their tendency to gain weight.
This was a good-quality study, but these and other limitations mean that we cannot conclude that a definite cause-and-effect relationship exists. As the researchers say, the possibility remains that these are chance findings. This study is worthy of being replicated in larger groups that take more detailed measures in mothers and their children.
Where did the story come from?
The study was carried out by researchers from the universities of Bristol and Glasgow and University College London. Funding was provided by the National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases, UK Medical Research Council, Wellcome Trust and University of Bristol. It was published in the peer-reviewed medical journal Circulation.
Generally, the Daily Mail accurately reported the findings of this research. However, the study has several limitations that mean its conclusions are not as clear cut as has been reported.
What kind of research was this?
This cohort study examined the association between the mother’s weight gain during pregnancy, weight before pregnancy and the child’s body fat and cardiovascular risk factors.
Though a cohort study is the best way to assess whether a particular exposure (in this case maternal weight gain in pregnancy) increases the risk of an outcome (whether the child is overweight), there are likely to be several confounding factors that could affect the relationship. These factors make it difficult to tell whether pregnancy weight gain directly causes higher body fat in the child.
What did the research involve?
This research studied participants from the Avon Longitudinal Study of Parents and Children (ALSPAC), which recruited 14,541 pregnant women living in Avon who gave birth in 1991-92. This analysis was restricted to single babies who were born at full term and survived to at least one year. This resulted in 12,447 mother and child pairs.
The sample was further restricted to women who agreed for their medical records to be examined and whose children attended a follow-up assessment at the age of nine. Full data on the mothers’ weight gain during pregnancy, and the child’s blood pressure, weight, height and total body fat measurements also had to be available, which resulted in 5,154 mother and child pairs (41% of the 12,447 total).
Blood samples were available (for blood cholesterol levels) for 3,457 children (28% of the total). Other measurements taken at the nine-year follow-up included the child’s BMI, waist circumference, cholesterol, and biochemical markers and hormones related to higher body fat and inflammation. Possible confounding factors that were considered were maternal age, how the child was delivered, total number of children, sex of the child, age of the child at outcome assessment, maternal pregnancy smoking and socioeconomic factors.
The mothers’ weight during pregnancy was obtained from their pregnancy records. The first weight measurement was subtracted from the last, to give the absolute weight gain. These measurements were compared to the recommended absolute gestational weight gain (GWG), calculated according to the mothers' pre-pregnancy BMI.
The recommended GWG (as given by Institute of Medicine [IOM] guidelines) is as follows:
- Underweight pre-pregnancy (BMI <18.5 kg/m2): 12.5–18kg absolute gain recommended.
- Normal weight (BMI 18.5–24.9): 11.5–16kg absolute gain recommended.
- Overweight (BMI 25–29.9): 7–11.5kg absolute gain recommended.
- Obese (BMI ≥30): 5–9kg absolute gain recommended.
Women were weighed on average 10 times during pregnancy, so in addition to absolute pregnancy weight gain, the women’s weight change according to their stage in pregnancy was also considered.
The relationship between the child’s measurements at nine years, the mother’s recommended GWG category and her weight change across pregnancy were then analysed using statistical modelling techniques.
What were the basic results?
Women who exceeded their IOM-recommended GWG were more likely to have a child who, at nine years of age, had higher BMI, waist circumference, total body fat and blood pressure. The blood tests showed them to also have lower levels of HDL (“good”) cholesterol, and higher levels of various biochemical markers and hormones that are related to higher body fat and inflammation (such as leptin, C-reactive protein and interleukin-6 levels).
Women who gained less than their recommended GWG tended to have children with lower BMI and waist circumference at age nine than those gaining the recommended amount. However, there were few differences for other child outcomes measured.
Further analysis indicated that greater pre-pregnancy weight was associated with greater child BMI, waist circumference and body fat and greater cardiovascular risk factors at age nine. When the researchers looked at weight gain across the stages of pregnancy and body fat in the child, they found that increased weight gain during early pregnancy (0–14 weeks) and mid-pregnancy (14–36 weeks), but not late pregnancy (after 36 weeks), was associated with an increase in child BMI, waist circumference and body fat. However, early pregnancy weight gain was not associated with greater cardiovascular risk factors in the child (as measured by the blood samples), while greater weight gain after 14 weeks appeared to be associated with an increase in these risk factors in the child.
How did the researchers interpret the results?
The researchers concluded that greater maternal pre-pregnancy weight and weight gain during pregnancy are associated with greater body fat in the child and adverse cardiovascular risk factors.
However, they suggest that before attempts are made to more strictly control weight gain during pregnancy, the risks and benefits for both the mother and child in the short and long term need to be assessed.
This large cohort study assessed the relationship between maternal weight gain during pregnancy and body fat and cardiovascular risk measures in children at age nine. Though the statistical modelling seems to show that mothers who put on more weight than recommended during pregnancy have more overweight children, a number of points should be considered when interpreting these findings:
- The large size of this cohort lends strength to the study's findings. However, only 41% of the total cohort was assessed, and blood samples measuring cholesterol and other cardiovascular risk factors were available for only 28% of children in the cohort. Inclusion of data from the remainder of the cohort could potentially affect the results.
- The analyses took into account confounders such as how the child is delivered, smoking in pregnancy, child’s sex and the parents’ socioeconomic status. However, other important lifestyle factors, such as diet and activity levels in both the mother and child, were not considered. This makes it difficult to say that the mother’s weight gain during pregnancy affects the child’s body fat rather than the lifestyle habits common to both. Also, mother and child are genetically linked, and shared genetic factors could affect their tendency for weight gain.
- The children were not assessed in the longer term. Levels of body fat and cholesterol at the age of nine may not indicate increased body fat and greater cardiovascular risk in adulthood.
- This cohort was collected in the early 1990s and only 7% of pregnant women in this cohort were obese. These figures may not be representative of the current rates of obesity almost two decades on.
As the researchers say, the possibility remains that these are chance findings. This study is worthy of replication in larger birth cohorts that take detailed pregnancy measures and outcome assessments in the child.