Why eldest child is unhealthier than siblings

• First-borns found to have greater chance of high blood pressure, being obese
• Mothers with pre-pregnancy hypertension more likely to have baby boy
First born children may have a head start in life, but they are heir to a host of other ailments. They are prey to unhealthy conditions, which may curtail their longevity.

From obesity to raised dietary fat and high blood pressure, being the eldest child is not always an advantage, a new study has found. While firstborn have the bonus of their parents’ undivided attention and resources, it can become a burden.

The stress of having to succeed at school, and meet career expectations, may be behind some of their health issues too, it is suggested.

The study was published in the journal Economics and Human Biology.

The research – one of the largest of its type – was based on the health records of almost 400,000 Norwegians over nearly a quarter of a century.

Lead author Professor Sandra Black of the University of Texas at Austin said: “Overall, we find that first-borns are less healthy in terms of physical markers such as blood pressure, triglycerides, and indicators of overweight and obesity.

“For example, compared to fifth-borns, first-borns are about five per cent points more likely to be obese and seven per cent more likely to have high blood pressure. So, unlike education or earnings, there is no clear first-born advantage in health.

“However, first-borns are about 13 per cent less likely to smoke daily than fifth-borns and are more likely to report good physical and mental health. Later-borns also score lower on well-being with fifth-borns being about nine per cent less likely than first-borns to report that they are happy. When we explore possible mechanisms, we find that early maternal investment may play a role in birth order effects on health.”

The study found that the probability of having high blood pressure declined with birth order and the largest gap is between first- and second-borns.

Second-borns are about three per cent less likely to have high blood pressure than first-borns; fifth-borns are about seven per cent less likely to have high blood pressure than first-borns.

They also found higher levels of high triglycerides – a form of dietary fat made by the liver – for first-borns than for second- or third-borns.

While first-borns are still taller – by just 1/8th of an inch over each succeeding child – they are also more likely to be overweight and obese. Compared to second-borns, first-borns are four per cent more likely to be overweight, and two per cent more likely to be obese. This may be down to their being breastfed for an extra two weeks , on average, it was suggested. Even being breastfed for two extra weeks, on average, has its drawbacks. However, the eldest child is more likely to be happy and consider themselves to be in good health Mental health generally declines with birth order.

Meanwhile, the gender of a woman’s future child may be linked to her blood pressure six months before she becomes pregnant, a preliminary study suggests.

A team of Canadian and Chinese researchers found that a higher pre-pregnancy blood pressure reading might be associated with a greater likelihood for delivering a baby boy. Conversely, lower blood pressure may favor the odds of giving birth to a girl.

But the researchers only found an association between pre-pregnancy blood pressure and a baby’s gender. They did not prove a cause-and-effect connection.

How might a mom-to-be’s blood pressure predict her baby’s gender? That’s not completely clear. The researchers suspect blood pressure may be related to the gender of those babies carried to term, not lost to miscarriage.

The study was published in the January 12 issue of the American Journal of Hypertension.

“When a woman becomes pregnant, the sex of a fetus is determined by whether the father’s sperm provides an X or Y chromosome, and there is no evidence that this probability varies in humans,” said study lead author Dr. Ravi Retnakaran.

He is an endocrinologist at Mount Sinai Hospital and the Lunenfeld-Tanenbaum Research Institute, both in Toronto. “But what is believed to vary is the proportion of male or female fetuses lost during pregnancy,” he said, referring to the risk for miscarriage or other pregnancy loss. In other words, the findings suggest either that lower maternal blood pressure shortly before pregnancy boosts the likelihood of carrying a female fetus to term, or higher blood pressure before pregnancy boosts the likelihood for carrying a male fetus to term, Retnakaran said.

But, he added, “This is an association — not cause and effect — between a woman’s blood pressure before pregnancy and her likelihood of delivering a boy or girl when she becomes pregnant.”

Retnakaran said researchers have previously observed that large “societal events” — such as natural disasters or an economic depression — appear linked to shifts in the boy-girl birth ratio. “We hypothesized that there are likely to be physiologic factors in a woman that relate to her likelihood of carrying a boy or a girl when she becomes pregnant,” he said.

The new study included just over 1,400 newly married women in Liuyang, China, and began in 2009. All the women indicated an intention to become pregnant within six months. All of them underwent full lab tests to record their blood pressure, cholesterol, triglyceride and glucose levels. These measurements were taken at an average of six months before pregnancy.

Once the women became pregnant, their health was tracked. All received routine obstetric care, including continual monitoring of blood pressure shifts, as well as the diagnosis of any complications throughout their pregnancies.

Ultimately, the study participants gave birth to 739 boys and 672 girls.

The study authors found that women who gave birth to boys had registered a higher pre-pregnancy systolic blood pressure (the upper number in a blood pressure reading) than women who gave birth to girls. Mothers of boys averaged about 113 mm Hg, versus mothers of girls who had an average near 110 mm Hg.

The researchers adjusted the data for maternal age, educational background, smoking history, obesity, and cholesterol, triglyceride and blood sugar levels. They found the blood pressure gap held.
Meanwhile, the study in the journal Economics and Human Biology, said that the sex of the first born did not play a part: “While there are some differences by gender, there is no strong systematic pattern and, in general, the coefficient sizes are similar across gender.”

Adding: “First-borns have higher average education and earnings than later-borns – so this is a plausible explanation for poorer health outcomes of later-borns. However, it cannot explain the fact that later-borns have better health along some dimensions such as having lower risk of high blood pressure and obesity. First-borns are, on average, lighter at birth than their siblings. Lower nutrient flow to first-borns in utero may affect their regulation of fat and cause them to store more fats in adulthood. This may partly explain the greater propensity of first-borns to be obese.

“The resulting catch-up growth of first-borns also leads to a greater tendency towards high blood pressure. Thus, the greater likelihood of high blood pressure and obesity for first-borns may be largely biologically determined.

“Another potential mechanism is personality. While the empirical evidence is not particularly strong, there is an established set of theories about birth order and personality.

First-borns are often perceived to be intense and career-orientated while later-borns are considered to be more laid back and creative .
“This provides a possible set of explanations for our findings about blood pressure. High blood pressure and triglycerides may be caused by the stress that results from this driven, competitive personality type.”