What does birth order have to do with weight? A lot, according to a new study. (Photo: GettyImages/Hanifa for Yahoo Health)

Simply being the firstborn sister may put a woman at higher risk of becoming overweight or obese down the road, new research published in the Journal of Epidemiology & Community Health suggests.

In the study, conducted at the University of Auckland’s Liggins Institute, Australian and Swedish researchers evaluated data on 13,406 pairs of sisters — not including twins — born between 1991 and 2009 from the Swedish Birth Register.

Scientists found that firstborn sisters were 29 percent more likely to be overweight later in life than their second-born siblings. They were also 40 percent more likely to be obese despite being just a little bit taller than their younger sisters. To remove other influences that may affect weight, such as environment and genetics, researchers looked specifically at sisters in determining whether firstborn status may contribute to weight issues later in life.

This new research seems to coincide with other observational studies conducted by Liggins Institute researchers of men and children that also showed a link between birth order and weight. These studies dug even deeper into the data on firstborns, which indicated being oldest was also associated with higher blood pressure and insulin resistance.

Related: What Being An Oldest, Middle, Or Youngest Child Says About You

Why does being born first mean getting the shorter end of the stick, health-wise? Although no research has confirmed a cause, lead study author Wayne Cutfield, a professor at the Liggins Institute, tells Yahoo Health it may have to do with a difference in blood supply to the placenta during the first pregnancy compared with later pregnancies.

The vessels are narrower during the initial pregnancy, says Cutfield via email. “This reduces the 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,” he writes.

Cutfield says being the oldest sibling is probably just a “small contributor” to the overall odds of becoming obese or overweight in later years. From his research, the differences in both obesity and insulin resistance between firstborns and later-borns is somewhere between 20 and 25 percent — a variation that is not great enough to make it be considered a major risk factor.

However, knowing your disease risk should always motivate you to make better decisions about your health, Cutfield says. Just as you might try harder to eat well-balanced meals and exercise regularly if you have a family history of heart disease or diabetes, with more research on firstborn health risks, this group could be similarly empowered to take charge of their well-being.

Are you like oldest child D.J. Tanner on “Full House”?  (Photo: Getty Images/ABC Photo Archives)

Being the oldest child in the family means going through a lot of firsts — and quite possibly failures — before future siblings come along.

First step. First word. First time-out.

The oldest children are often put through experimentation or test phases, when parents are still trying to figure out the goals they have for their children. While oldest children may have had a more strict — or at least watchful — upbringing, it also means they received a lot of attention because they had their parents all to themselves, Khadijah B. Watkins, MD, assistant professor of psychiatry in the Division of Child and Adolescent Psychiatry at Weill Cornell Medical College, tells Yahoo Health.

As a result, firstborns command more attention and approval from peers and colleagues and are more in tune with their surroundings.

Many world leaders are firstborn kids who are typically more driven and motivated, Watkins says. They are also extremely competitive, confident, organized, and strive for greatness. 

A Norwegian study found that firstborns had a slightly higher IQ than middle or younger siblings, supporting the fact that they want to excel at everything they do, from landing a successful job to simply planning a vacation. 

Are you an oldest child? You’re probably…

Power-driven

Successful

Competitive

Motivated

Strong

Responsible

Confident

Intelligent

Rule keepers

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Do you see some of yourself in Luke on “Modern Family”? (Photo: Getty Images/Peter “Hopper” Stone)

The last-born sibling — also known as the “baby” of the family — enters into the familial hierarchy at the bottom of the pyramid, but is typically the most spoiled of all the siblings.

By the time parents have their last child, they know the drill — changing diapers is a piece of cake and teaching their child how to ride a bike is second nature.

Parents are also more lenient about certain things and tend to intensify how much they coddle, spoil, and protect the youngest child. Last-borns start becoming familiar with people doing things for them, so they are typically more demanding and impatient when things don’t go their way.

Youngest children often want their own way in life and seek to tailor things toward their wants and needs, says Khadijah B. Watkins, MD, assistant professor of psychiatry in the Division of Child and Adolescent Psychiatry at Weill Cornell Medical College. 

“They’re always questioning the system and asking why things need to be that way,” Watkins tells Yahoo Health.

This rebellious attitude also leads them to be more free-spirited, outgoing, and prone to taking more risks.

Like firstborns, last-borns also tends to be highly motivated and driven. They’re competitive as well, but in most cases, they won’t directly compete with their siblings – they’re more likely to take a different course or even move to a new geographic location, Watkins tells Yahoo Health.

And youngest children know when to throw out a good joke or two — a recent study found that last-borns are more funny and easygoing compared to their older brothers and sisters. 

Are you a youngest child? You’re probably…

Rebellious

Competitive

Driven

Humorous

Risk taker

Outgoing

Demanding

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Your sibs influenced more than just your taste in music and movies! (Photo: ImageBrief)

Your parents brought you into this world, but that doesn’t mean they’re the only ones who have a big impact on your life. Whether or not you got along with your siblings when you were younger, there’s no questioning their impact on your personal growth and health — long after you’ve stopped sharing bunk beds.

Outside of the parental relationship, your relationship with your siblings is one of the longest you’ll have in your lifetime, says Elizabeth Ochoa, PhD, chief psychologist for the Department of Psychiatry at Mount Sinai Beth Israel.

Studies show that during adolescence, you can spend 10 to 17 hours a week with your brothers and sisters. It’s no wonder your characteristics, personality traits, and habits rub off on each other.

“There are many more positives to having a sibling versus negatives,” says Khadijah B. Watkins, MD, assistant professor of psychiatry in the Division of Child and Adolescent Psychiatry at Weill Cornell Medical College. They help shape, influence, mentor, and counsel each other, well into adulthood.

While sibling studies have been neglected in the family and child psychotherapy literature, Ochoa tells Yahoo Health, there is a growing awareness about the importance of having a brother or sister. Here are eight research- and expert-backed reasons your siblings are good for your health:

1. Siblings can help promote generosity.

It’s fairly common in families with multiple children for the older siblings to take on a mini-parent role, and younger siblings model after this loving, caring behavior. 

These characteristics from siblings can promote good deeds in the community, according to a study from Brigham Young University. Researchers found that siblings who felt loved and safe were more likely to reach out to help a neighbor or watch out for fellow classmates at school.

The spirit of good deeds that you learn from the sibling dynamic – cooperating and sharing with one another – may create a greater sense of generosity to the world at large, says Ochoa.

2. Siblings can help inspire passions.

Whether you’re the firstborn, middle child, or last-born of the family, personality traits according to your birth order play a role in discovering your passions.

Older siblings tend to be more motivated, driven, and successful, and as a result, middle children often are more creative to make a name for themselves. The youngest siblings are motivated and driven like firstborns, but they won’t compete with their siblings – they’re more likely to take a different course or move to a new geographic location. 

Parents’ categorization of their children plays a role as well, says Ochoa. If parents consider the firstborn to be the “smart one” in the family, the middle and youngest children might strive to be something more unique, like the “athlete in the family” or the “creative one.”

3. Sibling quarrels can teach conflict resolution.

Bickering with your brother or sister might have been the bane of your existence growing up, but it turns out scuabbling could actually play a role in the development of your own conflict resolution abilities.   

Sibling conflict teaches how to negotiate competition, share, listen properly, and communicate effectively, Ochoa says. 

4. Sibling relationships build loyalty and long-term friendship.

It’s an unspoken promise between siblings: You’ve got each other’s backs.

“There could be a biological push to relate to your siblings, since they share 50 percent of the parents genes,” says Ochao.

Even if siblings aren’t close growing up, many brothers and sisters develop stronger relationships with one another as they grow older. “Toward the 80- to 85-year-old [age range], some suggestions have been made that siblings allow you to enter old age with greater life satisfaction and less loneliness,” Ochoa tells Yahoo Health.

5. Siblings can have a positive impact on your weight.

“Jack and Jill went up the hill” may be part of a nursery rhyme, but this brother/sister duo knows the importance of physical fitness.

Research shows that if one sibling becomes obese, it increases the other sibling’s likelihood of obesity by 40 percent — also known as the “obese sibling.” But, that likelihood decreases the more active siblings become together.

Related: How Family Meals May Fight Obesity

6. Siblings help boost your immune system.

If you have a pretty strong immune system, you might have your siblings to thank.

“We’ve always known that having another sibling in the family decreases infections,” says Watkins. Studies show that exposure to germs, infections, and allergies may help boost the immune system of fellow siblings.

7. Siblings can help facilitate greater life satisfaction.

During adolescence, siblings have more of a sense of self-competency and greater life satisfaction, especially as they shift into adulthood.

“The importance of siblings and wellness and life satisfaction seem to become apparent at the point of developmental shifts,” says Ochoa.

Life hurdles — a parent’s illness or death, for example — also force siblings to unite and work to solve the problem cooperatively. “It allows them to come closer and support each other to come up with balanced solutions during these life shifts,” says Ochoa. 

Related: C’mon, Get Happy! 30 Easy Ways To Boost Your Mood

8. Your siblings might make you do better in school.

Little did you know that you had a built-in tutor who sleeps in the room across the hallway. 

A recent study found that an older sibling’s high test scores seemed to boost a younger sibling’s chance of landing a good grade.

Study researchers are calling it the “sibling spillover” effect, as siblings were found to mimic the academic goals and achievements of their brothers and sisters.

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