Latest news with #JorgeChavarro
Yahoo
20-07-2025
- Health
- Yahoo
It's a girl — again! And again! Why a baby's sex isn't random.
A baby's sex may not be up to mere chance. A study published Friday in the journal Science Advances describes the odds of having a boy or girl as flipping a weighted coin, unique to each family. It found evidence that an infant's birth sex is associated with maternal age and specific genes. Subscribe to The Post Most newsletter for the most important and interesting stories from The Washington Post. The findings challenge assumptions that birth sex is random. They mirror the results of similar studies in Europe that have also found that birth sex does not follow a simple 50-50 distribution. Scientists have long documented a global imbalance in which slightly more boys are born than girls. The new study examined the murkier patterns of birth sex within individual families. To do so, researchers at the Harvard T.H. Chan School of Public Health analyzed data from more than 146,000 pregnancies from 58,000 nurses in the United States between 1956 and 2015. They found that some families were more likely to have children of the same sex than would be expected if each baby had an equal chance of being a boy or a girl. Moms with three or more kids were more likely to have all boys or all girls than expected by chance. The study suggests that sex at birth follows a weighted probability and that biological influences may sway the sex of the child. 'If you've had two girls or three girls and you're trying for a boy, you should know your odds are not 50-50,' said Jorge Chavarro, the study's senior author. 'You're more likely than not to have another girl.' Researchers estimated that families with three girls had a 58 percent chance of having another girl, while families with three boys had a 61 percent chance of having a fourth boy. Maternal age is a key factor. Women who started having children after age 28 were slightly more likely to have only boys or only girls. Chavarro said this could reflect age-related biological changes that influence the survival of the Y chromosome carried by boys, such as increased vaginal acidity. Paternal factors could also play a role because maternal and paternal ages are often closely linked. But the study did not include data on fathers, which was noted as a limitation. Researchers also identified two genes associated with giving birth to only boys or only girls. 'We don't know why these genes would be associated with sex at birth, but they are, and that opens up new questions,' Chavarro, a professor of nutrition and epidemiology, said. Iain Mathieson, a professor of genetics at the University of Pennsylvania Perelman School of Medicine who was not involved in the study, said the genetic associations should be viewed cautiously. He said the study's genetic analysis was based on a relatively small sample and may be influenced by other factors, making the results more speculative until confirmed by further research. 'I don't find the genetic factors identified here particularly convincing,' Mathieson said in an email. The researchers also found that parents were more likely to have one boy and one girl than would be expected by chance, a pattern they believe reflects a tendency to stop having children once both sexes are represented. To reduce bias from such family planning decisions, they analyzed data after removing each woman's final child. They also excluded women who had experienced miscarriages or stillbirths to test whether pregnancy loss changed the results. They still found the same pattern: The odds of birth sex did not follow mere chance. The study suggests it may not have been so improbable for the fictional parents in the TV sitcom 'Malcolm in the Middle' to have five sons or for the Bennet family in 'Pride and Prejudice' to have five daughters. Even in history, patterns like this have drawn attention. King Louis VII of France, for example, remarried after his first two wives each gave birth to daughters, depriving him of a male heir. Chavarro said it might take years to fully understand why some families consistently have children of one sex, but this research is an important place to start. His team said future studies should explore how lifestyle, nutrition and exposure to environmental chemicals might affect these patterns. Certain factors such as race, natural hair color, blood type, body mass index and height were not associated with having children of only one sex. But the study sample was 95 percent White and made up entirely of nurses, a group that may have different occupational exposures or health patterns compared with the general population. David A. Haig, an evolutionary biologist at Harvard University who was not involved in the study, said it offers evidence that the probability of a baby's sex varies by family. 'Different families are flipping different coins with different biases,' Haig said. 'It speaks to something very intuitive and personal, even if the underlying biology is complex.' Related Content Family adopts a shelter dog — then learns he's the father of their late dog Can the Fed stay independent? Trump-era adviser may put it to the test. The Hubble telescope zooms in on the galaxy next door Solve the daily Crossword
Economic Times
19-07-2025
- Health
- Economic Times
New research challenges 50-50 myth: Biological sex of children may not always be random
Biological sex of children may not always be random A new study is turning our understanding of biological sex determination on its head. Long taught in biology class as a 50-50 genetic lottery between X and Y chromosomes, the sex of a child may not be entirely random. According to new findings published in Science Advances on July 18, individual families may have skewed odds toward consistently having either boys or girls — and age, genetics, and environmental factors may be involved. Led by epidemiologist Dr. Jorge Chavarro of Harvard T.H. Chan School of Public Health, the study analyzed data from the historic Nurses' Health Study , one of the largest and longest-running fertility databases in the world. Examining more than 58,000 pregnancies between 1956 and 2015, researchers looked at patterns of sibling sex within families, maternal age, and genetic data. While the broader population showed the expected close-to-even split between male and female births, something surprising appeared at the family level: some couples appeared more likely to have children of the same sex — and not by random one in three families in the study had children all of the same sex — all boys or all girls — a figure notably higher than simple probability theory would predict, especially in families with three or more researchers developed a statistical model suggesting that each couple might have their own "unique probability" of producing a child of one sex or the other. While this individual bias remains balanced out when looking at large populations, it could significantly affect outcomes within families. 'It's not that boys or girls are more common overall, but that the odds may not actually be 50-50 for everyone,' said Dr. Chavarro. The effect appeared even stronger in women who had their first child later in life. This suggests possible biological shifts over time, potentially influencing which type of sperm — X-carrying (girl) or Y-carrying (boy) — is more likely to fertilize the women age, for example, vaginal pH and cervical mucus composition change, potentially favoring sperm carrying specific chromosomes. X sperm tend to be larger and more resilient, possibly giving them a better chance at success in slightly more acidic environments, which can develop later in life. The study also uncovered two genetic loci that may be linked to the likelihood of having children of the same sex. However, these genes do not appear to be directly involved in known reproductive pathways — their role is mysterious and not yet fully understood. 'These are just initial hints,' Chavarro explained. 'They point to a potential genetic influence on sex bias, but don't yet explain the mechanism.' This finding opens new doors for genetic and evolutionary biologists to explore how heritable factors may interact with environmental and physiological variables to influence offspring findings, while intriguing, are not without controversy. Australian psychologist and behavioral geneticist Dr. Brendan Zietsch remains previously worked on a large Swedish study analyzing millions of birth records, which found no evidence of consistent sex patterns within families. He believes that the U.S.-based cohort (95% white, mostly nurses and health professionals) may not be representative enough to make broader study could reshape how we understand fundamental reproductive biology. For couples puzzled by having multiple same-sex children — and others praying for variety — the research may provide both insight and assurance. While conception still carries an element of chance, it may not be as random as previously also raises huge questions for future research: Could IVF or fertility treatments one day account for these biological skews? Could genetics or maternal physiology be guided to increase the chances of balancing the family tree?
Time of India
19-07-2025
- Health
- Time of India
New research challenges 50-50 myth: Biological sex of children may not always be random
A new study challenges the understanding of sex determination. It suggests families might have a bias toward having children of the same sex. The research, led by Dr. Jorge Chavarro, analyzed data from the Nurses' Health Study. Some couples were more likely to have all boys or all girls. Maternal age and genetics may play a role. Tired of too many ads? Remove Ads Same-sex siblings: More than mere coincidence? Tired of too many ads? Remove Ads Does parental age factor in? Genetics: A new frontier? Contrasting views from the scientific community Tired of too many ads? Remove Ads A new study is turning our understanding of biological sex determination on its head. Long taught in biology class as a 50-50 genetic lottery between X and Y chromosomes , the sex of a child may not be entirely random. According to new findings published in Science Advances on July 18, individual families may have skewed odds toward consistently having either boys or girls — and age, genetics, and environmental factors may be by epidemiologist Dr. Jorge Chavarro of Harvard T.H. Chan School of Public Health , the study analyzed data from the historic Nurses' Health Study , one of the largest and longest-running fertility databases in the world. Examining more than 58,000 pregnancies between 1956 and 2015, researchers looked at patterns of sibling sex within families, maternal age, and genetic the broader population showed the expected close-to-even split between male and female births, something surprising appeared at the family level: some couples appeared more likely to have children of the same sex — and not by random one in three families in the study had children all of the same sex — all boys or all girls — a figure notably higher than simple probability theory would predict, especially in families with three or more researchers developed a statistical model suggesting that each couple might have their own "unique probability" of producing a child of one sex or the other. While this individual bias remains balanced out when looking at large populations, it could significantly affect outcomes within families.'It's not that boys or girls are more common overall, but that the odds may not actually be 50-50 for everyone,' said Dr. effect appeared even stronger in women who had their first child later in life. This suggests possible biological shifts over time, potentially influencing which type of sperm — X-carrying (girl) or Y-carrying (boy) — is more likely to fertilize the women age, for example, vaginal pH and cervical mucus composition change, potentially favoring sperm carrying specific chromosomes. X sperm tend to be larger and more resilient, possibly giving them a better chance at success in slightly more acidic environments, which can develop later in study also uncovered two genetic loci that may be linked to the likelihood of having children of the same sex. However, these genes do not appear to be directly involved in known reproductive pathways — their role is mysterious and not yet fully understood.'These are just initial hints,' Chavarro explained. 'They point to a potential genetic influence on sex bias, but don't yet explain the mechanism.'This finding opens new doors for genetic and evolutionary biologists to explore how heritable factors may interact with environmental and physiological variables to influence offspring findings, while intriguing, are not without controversy. Australian psychologist and behavioral geneticist Dr. Brendan Zietsch remains previously worked on a large Swedish study analyzing millions of birth records, which found no evidence of consistent sex patterns within families. He believes that the U.S.-based cohort (95% white, mostly nurses and health professionals) may not be representative enough to make broader study could reshape how we understand fundamental reproductive biology. For couples puzzled by having multiple same-sex children — and others praying for variety — the research may provide both insight and assurance. While conception still carries an element of chance, it may not be as random as previously also raises huge questions for future research: Could IVF or fertility treatments one day account for these biological skews? Could genetics or maternal physiology be guided to increase the chances of balancing the family tree?
The Independent
19-07-2025
- Health
- The Independent
Your baby's sex at birth isn't a coin toss and can be influenced by these factors, new study claims
Gender reveals have become increasingly popular with families anxiously waiting to see if their newest addition will be born a boy or a girl. But despite popular belief, it's not a coin toss, according to a new study. A study published in the Science Advance journal Friday found families have a 'unique probability' when it comes to whether they will have a boy or a girl and there are certain factors influencing the odds. Researchers analyzed just over 58,000 women with two or more singleton births from 1956 to 2015, finding mothers with three or more kids were more likely to have children of the same sex. Rather than a 50-50 chance of having either a boy or a girl, families with three girls had a 58 percent chance of having another girl, and families with three boys had a 61 percent chance of having another boy. So why does this happen? Researchers point to the mother's age when she gives birth and genetics. Women who started having kids after the age of 28 were slightly more likely to have children of the same sex. The study also identified two genes associated with giving birth to only boys or only girls. Jorge Chavarro, the senior author of the study and a professor of nutrition and epidemiology at the Harvard T.H. Chan School of Public Health, told The Washington Post, 'We don't know why these genes would be associated with sex at birth, but they are, and that opens up new questions.' There are some limitations to the study including that there was no data on fathers. Additionally, an expert outside of the study warned about the study's genetic analysis. Iain Mathieson, a professor of genetics at the University of Pennsylvania Perelman School of Medicine, told the Post the analysis was based on a relatively small sample and could be influenced by other factors. Researchers in the study concluded more research was needed to study the extent to which the factors mentioned explain why some families are more likely to have children of the same sex. 'Until then, families desiring offspring of more than one sex who have already had two or three children of the same sex should be aware that when trying for their next one, they are probably doing a coin toss with a two-headed coin,' the authors wrote.
