According to a research published in the Journal of Epidemiology and Community Health women born first have more chances to be overweight. The study was conducted by researchers from New Zealand who used data from the Swedish Birth Register.
The research team analyzed around 13.500 pairs of sisters. They examined information about the weight of the women when they were born and during their first pregnancies. The researchers collected the data reported at their first prenatal visits when the participants were ten to twelve weeks pregnant.
According to the research firstborn sisters had 29 percent more chances of being overweight and 40 percent chances of being obese compared to their second-born sisters. It seems that at birth the older sisters weighted less. However in adulthood, when they reached their first trimester of pregnancy the firstborns presented a higher body index mass. The second-born sisters had a body index mass of 28.8percent, while the firstborns had a body index mass of 24.4. Even though the difference might not seem big it was statistically important.
The researchers involved in the study say that this is the largest study of this kind which was conducted exclusively on adult female sibling pairs. However these findings support other three studies which were conducted in the past. Those studies included adult males and females and children of both sexes.
The reason why women born first have more chances to be overweight is not clear. Scientists speculate that it has something to do with the placenta. Senior author of the study Dr. Wayne Cutfield from the University of Auckland in New Zealand explained that in later pregnancies the amount of blood supply to the placenta might increase and compared to them the blood vessels during the first pregnancy have a limited blood flow and are narrower.
This reduces nutrient supply, thus reprogramming the regulation of fat and glucose, so that in later life, the individual is at risk of storing more fat and having insulin that works less effectively,”Cutfield added.
The researchers drew attention to the fact that birth order is not the only factor which could increase the risk of obesity. Besides, being born first represents only a small contribution to an enhanced risk of obesity.
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