There is a huge amount of information online advising would-be parents on how they can choose whether they have a boy or a girl. 'Natural sex selection strategies' include adopting particular sex positions, eating certain foods or having sex at specific times to increase the chances of having a baby with the desired sex. What is the science behind these theories, and do they actually work?
How is a baby's sex determined? In humans, males have XY chromosomes whilst females have XX chromosomes. Dr Pini, a researcher at the Colorado Centre for Reproductive Medicine, explains how these different combinations are made during fertilisation: "Eggs always carry an X chromosome, so an X sperm will create a female (XX) and a Y sperm will create a male (XY). " Men produce roughly 50% X sperm and 50% Y sperm, so the chance of getting a boy or a girl are generally the same.
This picture shows a punnet square explaining the genetics behind sex determination
What are the claims out there?
There is a whole range of advice online on how to selectively conceive a boy or a girl. One of the most common is eating particular foods in order to promote a certain environment in the body (alkaline for boys, acidic for girls). Another widely spread claim is that the timing of intercourse in relation to ovulation is important. However there are many more obscure pieces of advice, covering areas such as sexual positions, temperatures and clothing. Are they true?
What scientific data is there? Advice regarding timing of intercourse and sexual positions are sometimes referred to as the 'Shettles Theory', named after a physician who wrote a book in 1971 detailing how to choose the sex of your baby. This was based on observations he made using microscopes that Y sperm are faster but more fragile than X sperm. These physical differences between X and Y sperm have since been refuted using more advanced technologies.
Studies outside of the lab, in which data was collected from pregnant women, have found mixed results. For example, one study collected urine samples from 221 women to carefully track their ovulation cycle and found that the timing of intercourse in relation to ovulation had no effect on the sex of the baby. Another study included over 3000 women but made a rough approximation of their cycles and found that there was a correlation. So far, pitfalls in study designs and conflicting conclusions means that there is no support for Shettles' claims.
Similarly, studies on the effect of mother’s diet on the baby's sex have also gained a lot of public interest but have not delivered clear conclusions. For example, one study asked 740 pregnant women what they had eaten the year before conception and during pregnancy. They found a correlation between eating larger breakfasts, particularly of cereals, the year before conception and having a boy. As so much information was collected on the mothers (not only diet, but health conditions, educational levels, medical history, etc.), it is likely that this one correlation occurred purely by chance. This, and other pitfalls, have meant that the results of this study do not solidly support natural sex selection methods.
As Professor Seidel, an expert in Developmental Biology, summaries: "There is no credible scientific evidence that having sex at different times of the cycle or that the acidic or alkaline environment of the reproductive tract have any significant effect on the sex of babies. There are hints that the sex may be skewed 2 or 3 percentage points by these practices, but such small differences are difficult to prove conclusively and are of essentially of no practical value."
Despite the range of claims that altering diet or timing and positions of intercourse can influence the sex of a baby, there is no scientific data to support them.
This article is based on 3 experts answers to the question: 'Are methods of natural gender selection effective?' Check out the Metafact website to see more answers to popular science questions.