I just found a really interesting article (Female gender pre-selection by maternal diet in combination with timing of sexual intercourse a prospective study, AM Noorlander, JPM Geraedts, JBM Melissen, Reproductive BioMedicine Online (2010) 21, 794 802). They did a prospective study combining timing of intercourse around ovulation combined with a special diet (high in calcium and magnesium, low in potassium and salt), and found an amazing result -- those who followed the diet and had the right levels of these minerals in their blood, and who followed the timing rule (BD 3 days before O) had an 80 percent sway towards girls. I had seen a lot of info on the in-gender forums, and always wondered what are the chances these things actually work, so thought that was interesting. Also interesting is that just considering the timing, those with BD 2 days before O had 80 percent sway to boys, those with BD 3 days before ovulation had 60 percent sway towards girls, those with BD 4 days before O had about 55 percent sway towards girl, and those with BD more than 4 days before O had 60 percent sway toward boys.
Anyway, all this got me very curious, and there was a table in the article on numbers of births and the sex of each child. I was able to calculate some really interesting statistics, based on the 935,288 families with children in Holland in 2003.
You know how everyone says that your odds of a boy if you already have 2 boys are really high? Or, others claim it is always 50/50? Well, here are the real numbers from those data:
|Odds of B, given no children||0.520|
|Odds of G, given no children||0.480|
|odds of B, given B||0.526|
|odds of G, given B||0.474|
|Odds of G, given G||0.478|
|Odds of B, given G||0.522|
|Odds of having BBG, given BB:||0.462|
|Odds of having BBB, given BB:||0.538|
|Odds of having GGB, given GG:||0.522|
|Odds of having GGG, given GG:||0.478|
It is well-known in demographic literature that the sex-ratio of boys to girls is actually higher than 50-50, so the 52/48 ratio for first births is totally on par with what is expected. If you have one boy already, your chances of another boy go up slightly from the already tilted -towards boys odds, to .526. If you have two boys already, your chances of another boy are almost 54 percent (and conversely, your chances for a girl are closer to 46 percent. And, if you have two girls, your chance of another girl are only about 48 percent.
(There were no data on higher order than this births cited in the study).
Just thought I would share!