3 cousins - likelihood of gender

[littleMissK]

Hi,

So my 2 sisters-in-law are ALL pregnant, due within 8 weeks (my mother in law already has repetitive strain from all the knitting she is doing!!) and I was wondering - what are the chances of us all having the same gender baby??

My maths is rubbish - I know we all have 50/50 chance of each... but beyond that I can't work it out. There's got to be some clever maths types here that could help me out!!

3 girls = 12.5%
3 boys = 12.5%
2 girls, 1 boy = 37.5%
2 boys, 1 girl = 37.5%

@nelly00

3 girls = 12.5% 3 boys = 12.5% 2 girls, 1 boy = 37.5% 2 boys, 1 girl = 37.5%

Thank you!!! it was hurting my head trying to work it out!

But aren't more boys born.... I swear I read somewhere that there is a higher percentage of boys due to evolution and then being risk takers 😂. Just to make it more complicated.

I recall reading a medical journal years back which looked at the fathers and granfathers families. If the father (and possible grandfather) had more male siblings, he was more likely to pass on Y chromosones and more likely to have more sons. If ,however, he was say 1 of a family of all girls, its more likely he will pass on X chromosones and have girls. I can't recall the stats, but might be another factor to think about.

Me and my 2 sister in laws were all pregnant at the same time, 2 months due date difference with each.

We all had boys

I was the last one to give birth and my mother in law was hoping for a girl. They're all so close now though it's brilliant.

Me and my 2 sisters in law have all had babies in the last year - all girls. And they are both pregnant again, also with girls, so 5 girl cousins born within 21 months!

Me and my two sister in laws were all pregnant, due within 3 months of each other and we had 2 girls and a boy.

In the last four years we've had seven (nearly eight as I'm expecting again in Autumn) babies born within in our family, all the first lot of grandchildren - so that was me and my three SILs either pregnant at the same time or within months of each other.

Five of those are babies were boys and three of them girls. All mine are the girls!

Me and my siblings had dc within 6 months of each other - had 1 girl, 2 boys between us. Overall my parents have 4 female and 2 male grandchildren. So didn't reflect the final outcome of the cousins.

The male birth rate is slightly higher than the female birth rate, at 105 boys to every 100 girls. Which means the first baby is 51.2% likely to be a boy and 48.8% likely to be a girl - standardised across all parental ages (older parents are more likely to have a girl, but that gets messy to factor in).

So you multiply the odds against each other - eg for 3 boys it's 0.512 X 0.512 X 0.512 = 0.134 or 13.4%, slightly higher than a 1 in 8 chance for all boys.

All girls is 11.6%, so slightly less than 1 in 8. The remaining 6 in 8 is a combination of boys and girls.

Once the first one is born, the odds of the next two being the same increase to (0.512 X 0.512) 26.2% for all boys or (0.488 X 0.488) 23.8% for all girls, as the first baby is now a known outcome.

I've been on maternity leave for 3 days from my job as a health statistician, can you tell I'm bored?

As mentioned above it's not as clear as 50-50 though if you want a basic percentage then just go with that one. It could get a lot more complex. For example, if a couple has two of the same sex then the chances are higher that the third child will be that sex.

@planbea this answer made me smile ☺

My three SILs all had girls within six weeks of each other