“The children of working mothers are more likely to be overweight than their peers”, the_ Daily Mail_ reported. It said that “latchkey kids have more freedom to eat sugary snacks and spend solitary afternoons slumped in front of the TV".
This large cohort study looked at data from 8,552 seven-year-old children in 1965, and compared it to 1,889 of their four to nine-year-old children in 1991. The clearest finding was an increase in childhood obesity/overweight in both girls and boys, and the most consistent association in both generations was between the mother's BMI and child BMI.
As reported, higher maternal employment seemed to have some association with higher child BMI in the 1991 cohort. However, there are likely to be several different factors involved in the increase in children's BMI, including diet and physical activity levels, which were not measured here. This is a good-quality study, but translating its findings to mean that more working mothers equates to more obese children is an oversimplification of the facts.
Where did the story come from?
The study was carried out by researchers from the University College London Institute of Child Health. It was funded by several sources, including the Great Ormond Street Hospital NHS Trust and a grant from the Institute of Child Health Special Project Initiative. The study was published in the peer-reviewed medical journal American Journal of Epidemiology.
The news has over-simplified the findings of this study. Although a positive link was seen between higher child BMI and maternal employment in the later generation, the association is not completely clear, and there are many other unmeasured factors that could be having an effect too. Therefore it is not possible to conclude that maternal employment is the only cause of higher childhood BMI.
What kind of research was this?
The purpose of this large British cohort study was to look at how risk factors for obesity have changed over time. The researchers compared the data from a population born in 1958 to that from any offspring they had by they time they were 33 years of age (in 1991). The parents’ data was collected when they were about seven years old, and the offspring data was collected when they were between four and nine.
The researchers used statistical models to look at the relationships between mothers and their offspring and their respective BMIs, and also looked at the factors that were associated at the time of the assessments.
Although the study can tell us how the prevalence of obesity has changed over time, it cannot tell us the reasons for this. The higher prevalence of obesity in the offspring cohort may be associated with a higher prevalence of working mums compared to the previous generation. However, it cannot be concluded that greater maternal employment is the only cause of the higher child BMI.
What did the research involve?
The first cohort included all people born during a particular week in 1958. About 17,000 of these people were then followed up on eight occasions until they reached the age of 45. In 1991, when the cohort was aged 33 years, 11,407 members were interviewed and of those who were parents, a random sample of one-third was selected. This gave about 4,300 children who formed the second, offspring cohort.
As all of the offspring had been born when their parent was aged 33 or younger, they formed two comparable cohort groups:
- Children born in 1958 who had been assessed in 1965 when they were seven years old and who had been born when their mother was under the age of 30 (8,552 members).
- The offspring cohort who were aged between four and nine years in 1991, and who had been born when their mother was under the age of 30 (1,889 members).
Height and weight measurements of the children were collected in 1991. Their parents were asked to complete a questionnaire providing extra information on their children, such as socio-demographic details, including whether they were married/cohabitating, employment status and housing, and information on various pre- and postnatal medical and lifestyle factors, e.g. whether the parents smoked, whether the baby was breastfed, etc. Similar information had been collected for the 1958 cohort, which also had their height and weight measured when they were seven. Their own parents BMI had also been measured (grandparents of the offspring cohort).
What were the basic results?
The authors found that the prevalence of overweight/obesity had increased by more than 50% between generations. Between the first cohort in 1965 and their offspring cohort in 1991, there had been a BMI increase of about 0.64 units among seven year old girls. For seven year old boys there was a BMI increase of about 0.23 units.
The researchers looked at numerous social factors to see whether they were associated with BMI in both cohorts, including maternal employment, housing and family size. A significant positive relationship was found between offspring BMI and their mothers’ BMI, i.e. there was a higher chance of the child being overweight/obese if their mother was. The association between child and maternal BMI had become more significant across generations. There was also a positive trend between increased BMI in the offspring cohort if their mother was in full-time employment; a relationship that was not seen in the 1958 cohort.
There was no clear relationship with any other factors, and they showed different associations in the two cohorts. For example, in the 1958 cohort, lower parental socioeconomic status was linked to lower child BMI; in the offspring cohort lower parental socioeconomic status was linked to higher child BMI.
Maternal employment had increased over the generations, and socioeconomic factors had improved.
How did the researchers interpret the results?
The researchers conclude that parental obesity, maternal employment and socioeconomic factors may play an increasing role in the childhood obesity epidemic.
This large cohort study looked at the BMI and socio-demographic status of 8,552 seven-year-old children in 1965, and then looked at 1,889 of their four to nine-year-old children in 1991. The researchers were interested in the change in obesity prevalence between generations, and to see whether it was related to any other factors.
The clearest finding from this research is that the prevalence of childhood obesity/overweight has increased, with a 0.64 BMI unit increase for seven-year-old girls and 0.23 units for boys. The researchers carried out statistical models to look at associations with other factors. They found a variety of trends and associations in both generations. Maternal BMI and child BMI had the most established association, which was consistent in both generations. Others were less consistent and some associations reversed. For example, in 1965 lower socioeconomic status was associated with lower childhood BMI, whereas in 1991 it was linked with higher BMI.
Unsurprisingly, maternal employment was found to increase from the first to the second generation. As reported, this higher maternal employment was also associated with a higher child BMI – a relationship which had not been seen in the first cohort. However, there are likely to be several different factors involved in this increase in BMI, including diet and physical activity levels – factors which have not been assessed for either of the cohorts in this study. This is a good-quality study, but translating it to mean that more working mothers equates to more obese children is an oversimplification of the facts.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 26 May 2010
Links to the science
American Journal of Epidemiology 2010; Advance Access published online on May 20