Latest news with #birthsex
Yahoo
23-07-2025
- Health
- Yahoo
Your odds of having a boy or girl might not be 50/50—here's what affects it
If you've ever joked that your family only makes boys, or been asked whether you're 'trying' for a girl, you're not alone. Many parents start to notice patterns in their children's birth sexes and wonder: Is it just chance, or something more? A massive new study from researchers at the Harvard T.H. Chan School of Public Health offers surprising clarity. After analyzing data from more than 146,000 pregnancies, scientists found that birth sex may be influenced by specific factors like mom's age or the sexes of older siblings. The odds, it turns out, might not be 50/50 after all. The study focused on biological sex assigned at birth, not gender identity, which can develop differently for each person. For families raising all boys or all girls, this could finally offer an explanation. And if you're hoping your next baby brings a different dynamic, this research offers a little insight—and maybe even a little peace. What the study found The research, published in Science Advances, drew on data from the long-running Nurses' Health Study. Rather than assume each baby has an even chance of being a boy or a girl, the researchers looked for patterns across families and found a few compelling ones. Here's what they found: Family patterns matter. Families with three or more boys were more likely to have another boy. The same held true for girls. After three girls, the odds of a fourth girl were about 58%. After three boys, the odds of a fourth boy were about 61%. Maternal age may play a small role. Women who began having children after age 28 were slightly more likely to continue having children of the same sex. This may relate to hormonal changes or shifts in vaginal pH. Genetics might be involved. Two genetic markers showed potential links to sex patterns, though researchers say these findings are early and need more study. While the study doesn't offer a formula, it suggests birth sex may follow subtle biological rhythms within families. Related: Why waiting to find out your baby's sex can be (surprise!) awesome Why some families may be more likely to have boys or girls At the population level, birth sex tends to balance out, with slightly more boys born worldwide. But zoom in on individual families, and a different picture can emerge. The researchers describe it like flipping a coin that's not perfectly fair. Some families seem to have a coin that lands on 'boy' more often; others, 'girl.' That tilt, it turns out, may be more biology than luck. '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,' The study's lead researcher Jorge Chavarro told Washington Post, 'You're more likely than not to have another girl.' The same is true for families with three boys. While the difference is subtle, the large size of the study makes these findings hard to ignore. Related: Choosing Your Baby's Sex. What doesn't make a difference Just as important, the study helped clarify what doesn't appear to influence a baby's sex. Researchers found no meaningful connection between birth sex and: Race or ethnicity Blood type Body mass index (BMI) Height Natural hair color Lifestyle habits, certain foods, or timing methods weren't included in this study and remain unproven. Since the study only included maternal data, paternal influence is still unknown. Related: Gender disappointment is real—and if you feel it, you're not alone What this means for families When you're raising three boys or three girls, people tend to notice. The questions start early—'Are you going to try again for a girl?' or 'Hoping for a boy this time?' and while often well-meaning, they can be exhausting. The truth is, many families didn't plan to end up with all boys or all girls. And according to this research, these streaks may reflect a biological pattern. That can bring a sense of relief. It shifts the narrative from personal choice or superstition to something more grounded: biology doing what it's wired to do. TikToks from #boymom and #girlmom creators show just how many families are joyfully navigating this path. And while those hashtags celebrate shared experience, it's worth remembering: every child is unique—personality isn't shaped by birth sex alone. Related: 'I dreamed of a daughter': This mom's emotional story of gender disappointment is resonating with parents Hoping for a mix? If you're hoping your next baby brings variety, you're not alone. Many parents do. This study doesn't offer a strategy, but it does offer perspective. The pattern in your family might gently continue. It's not a guarantee, but it's worth knowing. And whatever feelings come up—joy, longing, even disappointment—can all exist alongside deep love for the child you're welcoming. Related: To the mama preparing her heart for baby #2: You are ready The bottom line Science is still uncovering how families take shape. But this research offers reassurance for anyone who's noticed a pattern and wondered if it meant something more. Whether you're raising a girl gang, a pack of boys, or something in between, there's no 'right' mix. Each child brings their own kind of balance. And every family, in its own way, is already whole. Source: Science Advances. 2025. 'Is sex at birth a biological coin toss? Insights from a longitudinal and GWAS analysis' Solve the daily Crossword
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
