| Mother's Diet Influences Infant Sex
New research by the Universities of Exeter and Oxford provides
the first evidence that a child's sex is associated with the mother's
diet. The study shows a clear link between higher energy intake
around the time of conception and the birth of sons. The findings
may help explain the falling birth-rate of boys in industrialised
countries, including the UK and US.
The study focused on 740 first-time pregnant mothers in the
UK, who did not know the sex of their fetus. They were asked to
provide records of their eating habits before and during the early
stages of pregnancy. They were then split into three groups according
to the number of calories consumed per day around the time they
conceived. 56% of the women in the group with the highest energy
intake at conception had sons, compared with 45% in the lowest
group. As well as consuming more calories, women who had sons
were more likely to have eaten a higher quantity and wider range
of nutrients, including potassium, calcium and vitamins C, E and
B12. There was also a strong correlation between women eating
breakfast cereals and producing sons.
Over the last 40 years there has been a small but consistent
decline, of about one per 1000 births annually, in the proportion
of boys being born in industrialised countries, including the
UK, the USA and Canada. Previous research has also shown a reduction
in the average energy intake in the developed world. The 'obesity
epidemic' is largely ascribed to declines in physical activity
and differences in food quality and eating habits. There is also
evidence that skipping breakfast is now common in the developed
world: in the USA, the proportion of adults eating breakfast fell
from 86% to 75% between 1965 and 1991.
Dr Fiona Mathews of the University of Exeter, lead author on
the paper, said: "This research may help to explain why in
developed countries, where many young women choose to have low
calorie diets, the proportion of boys born is falling. Our findings
are particularly interesting given the recent debates within the
Human Fertilisation and Embryology Committee about whether to
regulate 'gender' clinics that allow parents to select offspring
sex, by manipulating sperm, for non-medical reasons. Here we have
evidence of a 'natural' mechanism that means that women appear
to be already controlling the sex of their offspring by their
Scientists already know that in many animals, more sons are produced
when a mother has plentiful resources or is high ranking. The
phenomenon has been most extensively studied in invertebrates,
but is also seen in horses, cows and some species of deer. The
explanation is thought to lie with the evolutionary drive to produce
Dr Fiona Mathews said: "Potentially, males of most species
can father more offspring than females, but this can be strongly
influenced by the size or social status of the male, with poor
quality males failing to breed at all. Females, on the other hand,
reproduce more consistently. If a mother has plentiful resources
then it can make sense to invest in producing a son because he
is likely to produce more grandchildren than would a daughter.
However, in leaner times having a daughter is a safer bet."
Although sex is genetically determined by fathers, mothers therefore
appear able to favour the development of one sex of infant rather
than another. The mechanism is not yet understood in mammals,
but it is known from IVF research that high levels of glucose
encourage the growth and development of male embryos while inhibiting
female embryos. In humans, skipping breakfast depresses glucose
levels and so may be interpreted by the body as indicating poor
environmental conditions and low food availability.
The group of women taking part in the study was representative
of the UK average in terms of the weight, health and lifestyle.
The findings showed no evidence of a link between a mother smoking
and drinking caffeine prior to pregnancy and the gender of her
baby. There was also no correlation between the body mass index
(BMI) of a mother and the sex of her child. Although this research
provides the first link between a human mother's diet and the
sex of her offspring, there is still no evidence that diet during
pregnancy, rather than around the time of conception, plays any
role in the sex of a fetus.
This research was published April 23, 2008 in the journal Proceedings
of the Royal Society B: Biological Sciences.
This study was funded by the Sir Jules Thorn Charitable Trust.