Throughout history a fast amount of research has been undertaken to understand the complexity of gender selection. This is what you need to know.
As you might know, the sperm of the man determines the sex of the child. The sperm contains 22 chromosomes as well as either an X- or a Y-chromosome. A woman's egg contains 22 chromosomes plus an X-chromosome. If the sperm that fertilises the egg carries an X-chromosome the baby will be XX and will become a girl. If the sperm carries a Y-chromosome the baby will be XY, a boy.
X + X = GIRL
But what factors influence which sperm will be able to fertilise the egg?
What factors would increase the chances of having a boy, which to have a girl?
A popular theory by Landrum Shettles in the 1960s assumed that male (Y) sperm are faster but more fragile than female (X) sperm. He thus promoted the idea that timing would have an impact on gender selection: To have a girl it was recommended to have intercourse a few days before ovulation. However, the theory is controversial with a lack of evidence. Other 'timing methods' (e.g. O+12) and empirical studies clearly question and contradict Shettles findings (e.g. Wilcox et al 1995, Gray et al 1998). Click here for more details.
So I researched further and found a far more convincing theory. It is about the influence of the mother's diet prior to conception. Observations that lead into this direction were already made in the 1930s when a German scientist called Herbst observed that the amount of male offsprings of Bonellia viridis in water increased significantly when he added potassium to aquarium water. Further research on animals and later humans confirmed the link between minerals and offspring gender.
In 1983 the French scientist François Papa published, after many years of research and consulting women who had the wish to conceive a certain gender, a book with the promising title "Boy or Girl. Choosing Your Child through your diet", suggesting that a diet high in sodium and potassium favours a boy while a diet high in calcium and magnesium favours a girl, claiming an above 80% success rate.
These findings were further investigated in the last decades, confirming success rates in the 70s% - 80s% (e.g. Noorlander et al 2010 - 81%). This means your odds of conceiving a girl are approx. 80:20 rather than approx. 50:50.
In 2016 a comprehensive study by scientists around Mahmoud Edessy with a 76% success rate for the wished gender came to the following conclusion:
"This study concluded that the diet method (relative excess of sodium and potassium ions) would favour the birth of males, while relative excess of Ca and Mg ions in the diet would favour the birth of females.
So by altering diet to include and exclude certain food,
the condition in the reproductive tract will be directly affected;
increasing the number of a particular sex."