GOTHENBURG, Sweden — Contrary to previous research and despite the obesity epidemic, median birth weights have remained stable across the past 6 decades, the results of a longitudinal population-based study from Sweden suggest.
Presenting the findings at the European Obesity Summit 2016, Jimmy Celind, a postgraduate student at the Sahlgrenska Academy at the University of Gothenburg, said that the results held even when researchers looked at specific percentiles of birth weight and took into account place of birth.
Although several studies from several countries have shown an increase in birth weights, Mr Celind pointed out that the majority "have had the time perspective of maybe starting in the 1970s and going on until the end of the 20th century."
He said that "this is only 2 or 3 decades," during which period the studies have seen a small trend for birth weights pointing upward. This rise has been "captured by all of them" and was believed to be part of an overall longer-term trend, he added.
Consequently, he and his colleagues were expecting also to see a rise in birth weights around the same period and that the upswing would continue. However, the long-term trend "is that not much is happening," he observed.
"We were also expecting to find the heaviest children to be heavy and more numerous; we cannot see that here."
Asked to comment, Jason Halford, PhD, of the European Association for the Study of Obesity and the University of Liverpool, United Kingdom, who was not connected with the study, described the findings as "interesting" and "very provocative."
He told Medscape Medical News: "What we need now to understand is whether this Swedish cohort that they're looking at is representative of what we're seeing in Europe as a whole or different territories within Europe.
"Obviously, different countries have a different time course in terms of the changes in prevalence of obesity and also perhaps windows in which they've looked at changes in birth weight as well."
Dr Halford also pointed out that birth weight is "only one of a number of early factors that may be related to future obesity," such as maternal diet, feeding practices after birth, and the time and form of weaning.
He added: "There's all sorts of critical windows, and so the relationship with early development and later obesity is a complex one, [with] lots of different factors in play."
Children of Gothenburg Studied
Mr Celind explained that they undertook the research as there is "a widespread belief that birth weights are increasing in the world, and this is at least partly because of the obesity epidemic and the increase in overweight in the population."
The team therefore examined data from the population-based BMI Epidemiology Study, which has collated information from Swedish Child Health Care and School Health Care centers on 400,000 children born in 1945 or later in Gothenburg to establish the role of childhood obesity on a range of diseases in later life.
Due to limited initial funding, the current analysis focused on 400 firstborn boys delivered every fifth year between 1946 and 2011, yielding 14 birth cohorts and a total of 5600 boys. The mean birth weight among these children was 3.53 kg.
Linear regression analysis indicated that the median birth weight was overall stable across the study period, at a beta value of -0.001 kg/year (P = .003), with only minor fluctuations during certain decades.
Restricting the analysis to children with both parents born in Sweden had no impact on the results. The fifth, 25th, 75th, and 95th percentiles of birth weight were also stable across the 65 years of the study.
Further analysis revealed, however, that there was a significant increase in the likelihood of children being born with a very low birth weight (defined as < 1.5 kg), at an odds ratio of 1.04 per year increment (P < .01). However, Mr Celind pointed out that likely "has to with the development in neonatal care" in more recent years.
There was no change in the likelihood of being born with a low birth weight (< 2.5 kg), at an odds ratio of 1.00 per year increment (P = .25), and only a slight reduction in the likelihood of a high birth weight (> 4.5 kg), at an odds ratio of 0.99 per year increment (P = .04).
Bigger Mums Should Equal Bigger Babies, but It's Not So
Other recent research reported by Medscape Medical News suggests that an important factor in the risk of childhood overweight and obesity is maternal gestational diabetes mellitus and excessive weight gain in pregnancy, irrespective of birth weight.
While Mr Celind believes that it "could be" that birth weight has less of an association with obesity and overweight than previously thought, it is well-known that "you have an increased risk of giving birth to a child with a high birth weight if you're obese and overweight."
Speaking to Medscape Medical News, he added: "We were expecting to see this shine through in our results, and we cannot really explain why not."
Addressing the possibility that a trend for increases in high-birth-weight children may have been offset by a large rise in the number of children being born with a lower birth weight, Mr Celind said: "We have looked at the stats in every way we could to get them to fit with the general opinion, but we can't do it all. So, no, I don't think so."
The authors report no relevant financial relationships.
European Obesity Summit 2016; June 2, 2016; Gothenburg, Sweden. Poster PP1.08.
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Cite this: No Impact of Obesity Epidemic on Birth Weight in Sweden - Medscape - Jun 03, 2016.