Wednesday May 18 2011
Diet is important for both the woman and unborn baby
The Daily Mail reported that there is a “life-long obesity risk of 'eating for two' in pregnancy”. The common advice, it says, is reinforced by the results of a study that followed women for a number of years.
The study used the body mass index (BMI) to follow 2,356 UK mothers during their pregnancy and once again 16 years later. Women were divided into groups depending on whether their weight gain during pregnancy was above or below US guidelines, with the researchers looking at how this related to their body shape and chances of being overweight 16 year later.
There are some limitations to the study meaning that its results should be interpreted with caution. For example, pre-pregnancy weight data was potentially inaccurate as it was not formally measured. Furthermore, there were no weight measurements taken between delivery and the 16-year follow-up point. This prevented the researchers from knowing if any excess weight was retained from pregnancy, or lost and gained again over time.
Overall the study highlights the importance of nutrition in pregnancy. The researchers say there is a 'window of opportunity' in pregnancy where paying attention to things like weight gain could help improve health outcomes for mothers and children later in life.
Where did the story come from?
The study was carried out by researchers from the University of Bristol and the University of Glasgow. It was supported by grants from several research institutes, including the Wellcome Trust in London, the US National Institutes of Health and the UK Medical Research Council.
The study was published in the peer-reviewed American Journal of Clinical Nutrition.
Generally the newspapers covered the story and its implications accurately. The Daily Mail gave useful examples of portion sizes, highlighting that the UK does not have specific guidelines for how much weight a woman should gain during pregnancy. The recommended gain for this study was based on guidance from the American Institute of Medicine.
What kind of research was this?
This was an analysis of data from a prospective cohort study. It looked at the links between a woman’s weight before pregnancy, her weight gain during pregnancy and her BMI, waist circumference and blood pressure measurements 16 years after the pregnancy.
The researchers explain that previous studies and one systematic review have already examined how child and maternal health may be linked to weight gain over the course of the pregnancy, called gestational weight gain (GWG). These previous studies had been restricted to three-year outcomes and looked at weight retention between pregnancies or links to breast cancer.
Reportedly, only one study looked at long-term weight gain, a piece of Australian research that looked at weight gain 22 years after pregnancy but only measured weight twice during pregnancy.
The researchers wanted to improve the evidence base on the subject by looking at more accurate measurements of weight during pregnancy and measuring outcomes at 16 years after the pregnancy.
As a large, population-based cohort design with women followed up over time, this study has used the best design for addressing these sorts of questions.
What did the research involve?
The data came from a study called the Avon Longitudinal Study of Parents and Children, a large, ongoing research project also known as the Children of the 90s study. This prospective, population-based cohort study had recruited 14,541 pregnant women living in Avon, England, with expected dates of delivery from 1991 to 1992.
This new maternal weight study excluded data on mothers who had given birth to twins and preterm babies. In total 12,976 mother and offspring pairs were available for inclusion in its analyses.
Midwives went through the medical records of the pregnancy and on average noted 10 separate recordings of weight in each mother’s notes. Other data was also collected, for example, mother’s age, number of previous babies, type of delivery (caesarean or vaginal birth), diagnosis of diabetes, blood pressure, and so on.
Questionnaires were used to collect a range of other data, such as socioeconomic status (based on parental occupation), height, pre-pregnancy weight, smoking during pregnancy, physical activity and diet in pregnancy, duration of breastfeeding and current smoking habits.
For their analyses, the researchers divided the women into three groups based on accepted recommended levels of gestational weight gain set out by the US Institute of Medicine:
- those with low GWG
- those within the recommended range
- those with high or above-average GWG
The recommended levels of weight gain in pregnancy are based on a woman’s BMI before pregnancy. These US guidelines state that:
- for women who are underweight before pregnancy (BMI less than 18.5) the recommended range of pregnancy weigh gain is 12.5 to 18kg (28-40lb)
- for women who are normal weight before pregnancy (BMI 18.5 to 24.9) the recommended range of pregnancy weigh gain is 11.5 to 16kg (25-35lb)
- for women who are overweight before pregnancy (BMI 25 to 29.9) the recommended range of pregnancy weigh gain is 7 to 11.5kg (15-24lb)
- for women who are obese before pregnancy (BMI more than 30) the recommended range of pregnancy weigh gain is 5 to 9kg (11-19lb)
The researchers modelled the link between a woman’s BMI and waist circumference 16 years after pregnancy and the GWG of her last pregnancy, adjusting the results for a range of things that might also influence the results. These included maternal age, offspring sex, social class, number of babies, smoking, duration of breastfeeding, current smoking and so on.
They had data for 2,356 women after the 16-year follow up.
What were the basic results?
Women with low GWG (according to the US Institute of Medicine definition) had on average lower BMI and waist circumference than women who had gained recommended weight levels during pregnancy. Women with a high GWG had a higher average BMI, waist circumference and blood pressure at 16 years.
After making their adjustments the researchers found that there was a three-fold increase in the chance that those with a high GWG were overweight and had central obesity at 16 years after the pregnancy compared with those who put on the recommended amount of weight during pregnancy.
Pre-pregnancy weight was positively linked to all outcomes; that is, the higher the woman’s pre-pregnancy weight, the greater her BMI, waist circumference and blood pressure 16 years after the pregnancy.
How did the researchers interpret the results?
The researchers say their results support initiatives aimed at ‘optimising pre-pregnancy weight’.
They add that the optimal GWG for each woman should take into account the balance of good and bad outcomes that have been linked with weight gain in pregnancy for both mothers and offspring. By this, they mean that under-nutrition can also be a risk for infants and that babies that are too small or too big at delivery can both be at greater risk of certain illnesses, as well as more likely to be born by caesarean section, for example.
They say it is important to recognise that identifying an ideal GWG has to reflect these competing risks.
This large study, with a long follow-up interval, has provided useful data for assessing what might be an ideal weight gain in pregnancy. There are some limitations and strengths to this study, some of which the researchers have discussed:
- The availability of repeat measurements of weight in pregnancy was a strength that allowed them to look at the links at the three trimesters of pregnancy. The strongest and most-consistent associations of GWG were with outcomes are in early and mid-pregnancy (conception to the 28th week of pregnancy).
- Pre-pregnancy weight was self-reported and this may have led to some inaccurate measurements. Also, some of the missing data for pre-pregnancy weight had to be estimated from the measured weight gains in pregnancy, which may have contributed to further inaccuracy.
- Apart from the measurements taken 16 years after birth, the researchers did not collect data on weight gain post-pregnancy. Therefore it is hard to assess whether the observed link of GWG with BMI later in life is due to excess weight retained from pregnancy or if it was gained later in life.
- Of 12,976 women originally included in the study, only 5,509 participated in the follow-up clinic at 16 years and 2,356 had their weight measured (a loss of 82% of the original study group). This degree of loss to follow-up is high, and it is not clear if the data on those that did not attend would have been different.
- If there is a link between GWG and later weight gain this study cannot confirm whether it is due to biological reasons or lifestyle factors.
Overall, this study does add to the knowledge about what might be an ideal maternal weight gain in pregnancy, but it should be remembered that there are risks for babies associated with under and over nutrition.
Diet in pregnancy should balance what is best for an expectant mother with what is best for their growing child. Also, there must be research into the causes for this weight gain, which could be due to biological changes following birth, lifestyle changes due to motherhood or some other unknown factor.