Firstborn girls are more likely to be overweight or obese in comparison to their youngest sisters, new research suggests.
The study, published in the Journal of Epidemiology and Community Health, set out to determine whether birth order influenced a woman’s height and weight in early life and adulthood.
Previous studies involving men found that firstborn sons are more likely to be taller and are at a greater risk for obesity, but data on whether a similar pattern exists in women is lacking.
Because of this, Professor Wayne Cutfield, of the University of Auckland in New Zealand, and his research team, felt that this area of research needed to be addressed.
“This is the fourth study we have done to characterize the health risks of firstborn in four different populations,” he said.
“If you look at the health risks of those that are firstborn, you find that firstborns are more insulin resistant than later borns, which is a risk factor for diabetes, and they have higher blood pressure than later borns.”
Using data from the Swedish Birth Register, the research team analyzed 303,301 women who were born between 1973 and 1988 who gave birth between 1991 and 2009. 206,510 of the women were firstborn or secondborn.
Of the 206,510, researchers identified 13,406 pairs of sisters, and assessed their weight and height at birth and at their first prenatal visit when they had reached the third month of their pregnancy.
The study found that firstborn sisters tended to weigh less at birth as compared to their younger sisters. However, their body mass index (BMI) was 2.4% higher during their first 3 months of pregnancy.
The results concluded that firstborn sisters were 29% more likely to be overweight and 40% more likely to be obese than their younger sisters.
“Our study corroborates other large studies on men, as we showed that firstborn women have greater BMI and are more likely to be overweight or obese than their secondborn sisters.”
Although the cause for this phenomenon is unknown, Curfield thinks that it may be because of more narrow blood vessels. In a woman’s first pregnancy, the blood vessels are narrower causing less blood to reach the placenta.
The researchers also made a possible connection to the lower birth weight in firstborns and the fact that firstborns are more susceptible to receive less than optimal nutrition in the womb.
“And this information has led to the hypothesis that firstborns were exposed to in utero compromise, which reprograms metabolism and the regulation of fat.”
Curtis feels as though his research could help empower firstborns to make healthier decisions throughout their lives, decisions that could reduce the risk of high blood pressure, obesity, and diabetes.
“There are minor health risks associated with being firstborn. I don’t want firstborns to think they will become obese or get diabetes or high blood pressure — it is a risk factor, and the risk of getting a disease is a combination of risk factors, not just a single risk factor.”
The study suggest that future studies with a large amount of subjects should research whether metabolic health issues are more prevalent in firstborn adults women.
This is not the first time birth order has been determined to affect certain characteristics in individuals.
Dr. Kevin Leman, a psychologist who has been studying birth order since 1967, says: “The one thing you can bet your paycheck on is the firstborn and secondborn in any given family are going to be different.”
A small study from New Zealand discovered that firstborns are at higher risk of developing heart disease and diabetes.
Firstborns often tend to be more rebellious, conscientious, structured, cautious, and controlling than their younger siblings. They have also been found to be the highest achievers in the family.
This personality is based on family dynamic and not pre-natal conditions. Because of the mass amount of control and attention given to them by their parents, they become over-responsible, well-behaved, smaller versions of their first-time parents.
Research has also found that firstborn children not only tend to have higher I.Q.’s than their younger siblings, but are also more likely to be the most ambitious and well-qualified of all the siblings. Their younger siblings tend to excel more in sports or the arts.
Firstborns also tend to be more conservative than their secondborn siblings. Research suggests these differences also stem from the family dynamic.