When a woman is pregnant, is the chance of having a girl or boy equal 50/50? Or are there like some biology stuff determining the odds?
In: Biology
It’s 50/50. While it has been believed or suggested that there might be a genetic disposition for a guy to lean one way, [genetic ](https://link.springer.com/article/10.1023/A:1020555101059)and [population ](https://www.cnn.com/2020/02/18/health/boys-girls-run-in-families-wellness-scn/index.html)studies have been unable to support this notion. As far as we can tell, it is entirely 50/50 random chance.
I wouldn’t say biology…but certainly genetics come into play. If you have two parents with a predisposition to create female offspring you’ll get girls. If those same parents mate with different partners the outcome will be diffeeent
So no; not every pregnancy has a 50/50 chance of boy or girl outcome
https://www.wtamu.edu/~cbaird/sq/mobile/2015/05/05/is-it-completely-random-whether-a-baby-is-a-boy-or-a-girl/
So we all have two things inside us that determine if we are either boys or girls. Girls have XX and boys have XY.
When humans reproduce the baby gets one from each parent. Girls can only give an X, since that’s all that they have. Boys can give an X or a Y. Since all boys have one X and one Y that means they have a 50/50 chance of giving either.
If the boy gives an X the child will be a girl, if the boy gives a Y the child will be a boy.
There were some findings in the 1960s suggesting that “Y-bearing” and “X-bearing” sperm cells had different sizes and swimming movements that might affect their fertilization chances, but recent work with better microscopes says the differences are subtle to the point of insignificance.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1440662/
https://www.frontiersin.org/articles/10.3389/fcell.2019.00388/full
Genetically speaking a typical XY dad should produce 50% X sperm and 50% Y sperm (give or take a few with nondisjunction abnormalities) and if it’s true that they swim equally well, then other differences could be
* sperm survival rates in the vas deferens
* sperm survival rates in the vagina/uterus/fallopian tubes
* sperm getting-through-the-egg-membrane capability
* fetal survival rates
* fetal health
* sex selection by parents
…and that’s all the other hurdles I can think of, on the way to obtaining a birth certificate.
[https://en.wikipedia.org/wiki/Human_sex_ratio](https://en.wikipedia.org/wiki/Human_sex_ratio)
It is slightly weighted toward boys. Estimated at around 105 boys to every 100 girls. There are many competing theories as to why this is.
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Yes, it is biology stuff. A woman has XX sex chromosomes and a man has XY†. Egg cells and sperm cells are special because they only have half of the genes needed to make a person, they combine and now you can start down the path to a baby.
Since eggs always have X, how do sperm choose? Sperm are made in pairs, by splitting a cell. This gives one X sperm and one Y. At this point the odds are always 50/50. But there are many steps, as others have described, between production and fertilization.
† It’s actually a little more complex as there are unusual conditions with 3 sex chromosomes, but that’s a statistically unlikely edge case and beyond the depth of ELI5.
It very slightly favors boys as least when it comes to live births. The reasoning behind this is unclear. It’s possible this is an evolutionary trait because Paleolithic males died more often hunting or in warfare so the slightly higher birth rate compensated. Another theory is that there is something inherently different about male and female sperm causing this. The only known different is that the male has an XY chromosome pair instead of an XX pair. The Y chromosome is basically an X chromosome with one section missing so there’s one theory that this sperm is carrying slightly less weight and is therefore slightly lighter and faster when it swims.
So somewhat equal but factors can affect the odds, for example higher testosterone in the father produces more girls, estrogen, more boys. (This is like a 1% difference)
some polutants in environment can probably influence the ratio – https://www.theguardian.com/environment/2021/dec/02/sex-ratio-of-babies-linked-to-pollution-and-poverty-indicators
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