“Overweight mothers-to-be could be condemning their unborn children to decades of ill health,” the Daily Mail reported.

The news is based on the results of large, long-term research that examined mothers’ body mass index (BMI) before and during pregnancy and how this was linked to various indicators of their children’s health when their children reached 32 years of age. These indicators included BMI, waist size and levels of fat and sugar in the blood, which are associated with the risk of conditions such as diabetes and heart disease.

Researchers found that higher maternal BMI before pregnancy was associated with increased BMI in children, as well as larger waistlines, raised blood pressure and increased blood levels of insulin and fat. Greater maternal weight gain during pregnancy was also associated with increased BMI, waist size and levels of fat in the blood.

This study adds to a growing body of evidence that mothers’ weight before and during pregnancy may affect a range of health-related factors in their children, perhaps even in the long-term. That said, this study’s design means that on its own it cannot prove that a mother’s weight or weight gain during pregnancy are responsible for the health effects seen in their grown-up children. For example, many complex environmental, social and genetic influences are known to determine who develops obesity.

Although the long-term effects of maternal weight are unclear from this study, excess weight is known to increase the risk of complications during birth, as well as making it harder to conceive in the first place. This study highlights the importance of maintaining a healthy weight for these reasons, rather than the potential long-term ones.

The Daily Mail also reported that “concern about the issue is so high that British doctors have started to medicate babies in the womb.” The newspaper appears to be referring to ongoing research into treating women for high blood sugar during pregnancy. This valuable study primarily aims to treat mothers, rather than their unborn babies, to cut potentially dangerous complications, rather than to improve their children’s long-term health.

Where did the story come from?

The study was carried out by researchers from the Hebrew University-Hadassah in Israel and the University of Washington in the US. It was funded by the US National Institutes of Health and the Israeli Science Foundation. The study was published in the peer-reviewed medical journal Circulation.

The story was accurately covered by the Daily Mail. However, it should be noted that this study measured factors that can contribute to various diseases, but did not determine the rates of negative health outcomes such as heart attacks, diabetes and strokes mentioned in the news coverage.

What kind of research was this?

This cohort study examined how mothers’ BMI and weight changes during pregnancy were associated with various markers of disease in their children once they reached adulthood. The disease markers of interest were waist circumference, BMI, blood pressure and levels of glucose, insulin, fats and lipoproteins in the blood. These were measured once the children reached 32 years of age. Maternal BMI and weight changes during pregnancy were reported by mothers in interviews conducted by nurses while they were in hospital after having their baby.

This is the ideal study design to examine a possible association between maternal weight and children’s health. The study’s strengths also included its large size and long follow-up. However, this type of study can only find associations between factors, and cannot prove a cause-and-effect link. This is because researchers cannot exclude the possibility that another factor is responsible for the association seen.

What did the research involve?

This research drew on data from a large, long-running study called the Jerusalem Perinatal Study. The research collected the following information on births in Jerusalem between 1974 and 1976:

demographic and socioeconomic information

medical conditions of the mother during current and previous pregnancies and gynaecological history

smoking status of the mother

height, pre-pregnancy weight and end-of-pregnancy weight of the mother

child’s birth weight and gestational age

This information was obtained from maternity ward logbooks, birth certificates and interviews with mothers while they were hospitalised after having their baby.

In this study, a sample of 1,400 individuals born during this period was interviewed and examined again between 2007 and 2009 (when they reached 32 years of age). Individuals who had been born as part of a multiple birth, who were premature or who had congenital malformations were excluded. The researchers collected data on:

height

body weight

waist circumference

blood pressure

levels of glucose, insulin and fats in the blood

The researchers looked at associations between maternal pre-pregnancy BMI and weight gain during pregnancy and children’s outcomes at 32 years of age. During their calculations, they accounted for gender, ethnicity and other factors that could explain any relationship seen, including:

how many previous pregnancies a mother had had

the mother’s age at the birth

maternal smoking, and smoking status of the children as adults

socioeconomic status

maternal education, and education of the children

maternal medical condition

the children’s birth weight and gestational age

physical activity of the children

What were the basic results?

The researchers found that greater maternal BMI before pregnancy was associated with the following factors in grown-up children at the age of 32:

increased BMI

increased waist circumference

increased blood pressure

increased blood levels of insulin and fat

lower levels of high-density lipoprotein cholesterol

These associations were independent of weight gain during pregnancy (i.e. were evident regardless of how much weight a mother gained during pregnancy).

Greater weight gain during pregnancy was associated with:

increased BMI

increased waist circumference

increased blood levels of fat

When calculating these various associations, the researchers split mothers into four evenly sized groups, based on their pre-pregnancy BMI. They found that, on average, the adult children of women in the group with the greatest BMI (maternal BMI more than 26.4kg/m2) went on to have a BMI of five units (kg/m2) higher than the children of mothers in the lowest quarter (maternal BMI less than 21.0kg/m2).

How did the researchers interpret the results?

The researchers concluded that “maternal size both before and during pregnancy is associated with cardiometabolic risk factors in young adult offspring.” In other words, mothers who have a high BMI before pregnancy or who gain a lot of weight during pregnancy are more likely to have children who have risk factors for various metabolic and heart-related health problems in adulthood.

The researchers added that these associations appear to be driven mainly by children’s body fat during adulthood.

Conclusion

The link between pregnant women’s weight and the health of their children has been in the public eye several times in recent months, with high-profile news stories questioning whether our mothers can “programme us to be fat” and the need to “treat babies for obesity while still in the womb”.

This latest study analysed potential links between mothers’ excess weight around the time of pregnancy and “cardiometabolic risk factors” in their children decades later. Cardiometabolic risk factors are factors such as raised BMI and blood sugar, which signal that a person has a higher risk of conditions such as diabetes and heart disease.

The study found a long-term relationship, with higher maternal weight (assessed using BMI) and greater weight gain during pregnancy associated with a number of factors in the mothers' children at the age of 32. These included increased BMI, waist circumference, blood pressure and blood levels of insulin and fats, and decreased levels of high-density lipoproteins (“good cholesterol”) in children. As the authors state, this study “adds to and extends accumulating evidence” of this relationship, as similar findings have been reported in other studies.

This study has shown associations between maternal weight and children’s later health, but it cannot show cause and effect. This is because it cannot rule out the possibility that another factor is responsible for the association seen. Also, both pre-pregnancy weight and weight gain were not directly measured but were reported by mothers in interviews conducted by nurses after delivery. This may have led to some inaccuracy in the calculation of BMI and makes the results less reliable.

The average pre-pregnancy BMI of women in this study was 24kg/m2 (within the healthy range) in mid-1970s Jerusalem. This population may not be typical of pregnant women in the UK today.

In addition, the exact mechanism by which maternal pre-pregnancy weight and weight gain during pregnancy might cause increased levels of cardiometabolic risk factors in children remains to be determined. Several mechanisms, including shared genetic and environmental characteristics or changes caused by exposures in the womb, have been proposed, although none is completely clear.

Analysis by Bazian

Edited by NHS Website