Twelve DNA areas 'linked with the age at which we have our first child and family size'

Lead author Professor Melinda Mills, from the Department of Sociology and Nuffield College at the University of Oxford, comments: 'For the first time, we now know where to find the DNA areas linked to reproductive behaviour. For example, we found that women with DNA variants for postponing parenthood also have bits of DNA code associated with later onset of menstruation and later menopause. One day it may be possible to use this information so doctors can answer the important question: "How late can you wait?" based on the DNA variants. It is important to put this into perspective, however, as having a child still strongly depends on many social and environmental factors that will always play a bigger role in whether or when we have babies.'

The study shows that DNA variants linked with the age at which people have their firstborn are also associated with other characteristics reflecting reproduction and sexual development, such as the age at which girls have their first period, when the voice breaks in boys, and at what stage women experience their menopause.

First author Nicola Barban, from the Department of Sociology and Nuffield College at the University of Oxford, comments: 'Our genes do not determine our behaviour, but for the first time, we have identified parts of the DNA code that influence it. This is another small piece to understanding this very large jigsaw puzzle.'

The researchers calculated that variants in the 12 areas of the DNA together predict less than 1% of the timing at which men and women have their first child and of the number of children they have in the course of their lifetime. The paper says that while these numbers seem 'extremely small', their modelling shows that in some cases when the variants are combined, they can be used to predict the probability of women remaining childless. Importantly, by examining the function of the 12 DNA regions and the genes in these regions in detail, the researchers have identified 24 genes that are likely to be responsible for the effects of the 12 DNA variants on reproductive behaviour. Some of these genes were already known to influence infertility, while others have not yet been studied.

According to study co-authors Professor Harold Snieder from the University of Groningen and Associate Professor Marcel den Hoed from Uppsala University, 'an improved understanding of the function of these genes may provide new insights for infertility treatments'.