{"id":272830,"date":"2025-07-18T19:32:12","date_gmt":"2025-07-18T19:32:12","guid":{"rendered":"https:\/\/www.europesays.com\/uk\/272830\/"},"modified":"2025-07-18T19:32:12","modified_gmt":"2025-07-18T19:32:12","slug":"having-only-boys-or-girls-sometimes-runs-in-the-family-heres-why","status":"publish","type":"post","link":"https:\/\/www.europesays.com\/uk\/272830\/","title":{"rendered":"Having only boys or girls sometimes runs in the family. Here’s why"},"content":{"rendered":"

There may be a genetic reason why some women give birth to only girls or boys. Here’s why<\/p>\n

Women who have had multiple children of the same sex are more likely to have another baby of the same sex, a new study has found.<\/p>\n

Maternal age and genetics could be \u2018weighting the coin toss\u2019 for some couples, rather than every child having a truly random 50\/50 chance of being a boy or a girl.<\/p>\n

The study also showed that older mothers were more likely to have children of the same sex,<\/a> and revealed two genes that could increase the likelihood of having all female or all male children, respectively.<\/p>\n

A child\u2019s sex at conception is determined by whether the sperm carries an X or Y chromosome, which should mean it\u2019s a perfect coin flip over whether a child will be a boy or a girl.<\/p>\n

However, PhD student Siwen Wang<\/a> from Harvard University noticed this didn\u2019t always seem to be the case.<\/p>\n

\u201cThis project actually began with casual conversations among co-authors and friends about families we knew who had all boys or all girls,\u201d said Wang to BBC Science Focus.<\/p>\n

\u201cIt came up often enough that we started wondering: is it really just chance? Or is there a biological reason some families keep having children of the same sex?\u201d<\/p>\n

Wang and her colleagues drew on information from more than 58,007 women who had given birth to two or more children, checking if there were more families with siblings of all the same sex than you might expect due to random chance.<\/p>\n

They found that if a couple had already had three boys, they had a 61 per cent chance of having another boy. Similarly, there was a 58 per cent chance of having another girl after having three girls.<\/p>\n

The study identified a few factors that potentially tipped the scales in favour of all girl or all boy families.<\/p>\n

\u201cWomen who had their first child after age 28 had about a 10 per cent higher chance of having only boys or only girls, compared to those who started before age 23,\u201d said Wang. \u201cSo it\u2019s not a huge shift, but it\u2019s statistically significant.\u201d<\/p>\n

Though the study didn\u2019t look into what might be causing this link, Wang did speculate on a few theories.<\/p>\n

\u00a0\u201cAs women age, they experience physiological changes such as shorter follicular phase and lower vaginal pH,\u201d she said.<\/p>\n

The follicular phase is the first stage of the menstrual cycle and tends to favour the survival of Y-chromosome sperm. Lower vaginal pH, however, favours X-chromosome sperm.<\/p>\n

\u00a0\u201cThese effects may differ from woman to woman, so ageing may tip the balance toward one sex or the other, depending on their specific biology,\u201d said Wang.<\/p>\n

\"ThreeFamilies of all girls or all boys are more common than mere chance would suggest – Credit: Getty Images\/pixdeluxe<\/p>\n

Wang also suggested another possible connection.<\/p>\n

\u201cOlder maternal age is usually highly associated with older paternal age. But unfortunately, we did not have paternal data to explore this aspect in our study,” said Wang.<\/p>\n

The researchers were also able to obtain genetic<\/a> information for 7,530 women included in the study, looking for any relevant genetic markers. They found two \u2013 SNP NSUN6, which was associated with all female offspring; and SNP TSHZ1, which correlated to all male.<\/p>\n

Wang also looked into whether behavioural factors could be creating such runs of single-sex children, such as couples who keep having boys continuing to have children until they have a girl, and vice versa. \u00a0<\/p>\n

\u201cWe ran analyses where we excluded the last birth in each family, which is the one most likely to be influenced by parents stopping once they\u2019ve had both a boy and a girl. Even after doing that, we still see strong same-sex clustering,\u201d said Wang.<\/p>\n

About our expert<\/p>\n

Siwen Wang is a PhD student in Nutritional Epidemiology at Harvard T.H. Chan School of Public Health. She investigates how nutrition, lifestyle, and psychosocial factors affect maternal and child health.<\/p>\n

Read more<\/strong><\/p>\n","protected":false},"excerpt":{"rendered":"There may be a genetic reason why some women give birth to only girls or boys. Here’s why…\n","protected":false},"author":2,"featured_media":272831,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3846],"tags":[267,70,16,15],"class_list":{"0":"post-272830","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-genetics","8":"tag-genetics","9":"tag-science","10":"tag-uk","11":"tag-united-kingdom"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@uk\/114875907708440567","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/272830","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/comments?post=272830"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/272830\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media\/272831"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media?parent=272830"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/categories?post=272830"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/tags?post=272830"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}