Fertility treatment may alter the gender balance

Published on 09/29/10

New research published in BJOG shows that certain types of assisted reproductive technology (ART) used to treat infertility lead to more male than female babies being born.

Researchers in Australia studied all live births following fertility treatment in clinics in Australia and New Zealand between 2002 and 2006. 13,368 babies were born to 13,165 women who underwent single embryo transfer (SET) in that period.

The study: Dean J, Chapman M, Sullivan E. The effect on human sex ratio at birth by assisted reproductive technology (ART) procedures – an assessment of babies born following single embryo transfers, Australia and New Zealand, 2002–2006. BJOG 2010; DOI: 10.1111/j.1471-0528.2010.02731.x.

In their study, the University of New South Wales researchers examined the effects of two different types of ART procedure: ‘standard’ IVF and ICSI (intracytoplasmic sperm injection, where the sperm are not motile and have to be injected directly into the egg), the stage of embryo development at the time of transfer – blastocyst (four days after fertilisation) or cleavage (two to three days after fertilisation), and the type of embryo used (fresh or thawed). The study was undertaken to find out if there is a link between the type of ART used and the sex ratio at birth (also known as the secondary sex ratio - SSR), defined as the proportion of males in all babies born alive. The authors note that deliberate sex selection is banned in Australia, and all fertility clinics comply with national guidelines.

In general the natural SSR varies across the world, and previous research has shown that the SSR from ART also varies. Researchers have speculated whether in the case of ART this is a result of the same natural causes that operate on spontaneous conceptions (eg. environmental or biological circumstances) or a direct consequence of ART.

Over the years there has been a big increase in the proportion of births from the use of ART, so it has become important to check that this does not result in a serious imbalance of male to female children.

In this study, the SSR for babies born from single embryo transfer (SET) was 51.3%, which is comparable to the SSR for the general Australian population (51.5%). However, the results also show that specific ART regimes do indeed influence the SSR. There were fewer male babies born after ICSI SET (50%) and more after IVF SET (53%). The stage at which the embryo was transferred had even more effect; 49.9% for cleavage stage SET and 54.1% for blastocyst SET.

The highest SSR was in the IVF SBT transfer group – 56.1% males – and the lowest in the ICSI cleavage SET group – 48.7%.

Jishan Dean, co-author of the study from the Reproductive and Perinatal Epidemiology Research Unit, School of Women’s and Children’s Health at the University of New South Wales said, “This is the first national population-based study to assess the impact of ART procedures and treatment practices on the human sex ratio at birth. The results from this study demonstrate that a particular ART procedure or treatment course can alter the probability of having a male baby from the natural sex ratio at birth in Australia of about 51.5%, to a low of 48.7% (a decrease of 2.8%) or a high of 56.1% (an increase of 4.6%), depending on the procedure used.

“This study provides impetus for further research into the underlying causes and or associations which determine the survival of different sexes of embryos or conceptuses in pregnancy.

“At an individual patient management level it is acknowledged that the type of clinical problem decides the most appropriate ART management plan, whether it is IVF or ICSI. The result of this study should therefore not be used as a de facto tool for sex selection.”

Professor Philip Steer, BJOG editor-in-chief said “In the general population, the secondary sex ratio (SSR) has been shown to vary over time and according to external factors such as in times of hardship, eg. famine or war. This could be nature’s way of balancing the sexes, ensuring the survival of the human species.

“This new study has revealed some interesting findings about the SSR after fertility treatment which may have future implications for public health. We know that already in some parts of India and China, a higher proportion of male babies have been born/survive because of parents deliberately choosing the gender of their baby, and this is leading to significant social problems with some men being unable to find a wife. It is important that we don’t allow such imbalances to occur unintentionally, simply because we have neglected to study the factors that influence the SSR ratio in the increasing proportion of the population who use ART.”

To speak to Professor Steer, please call 020 7772 6446. To contact Ms Dean, please call +61-413947688 or email j.dean@unsw.edu.au.

The study: Dean J, Chapman M, Sullivan E. The effect on human sex ratio at birth by assisted reproductive technology (ART) procedures – an assessment of babies born following single embryo transfers, Australia and New Zealand, 2002–2006. BJOG 2010; DOI: 10.1111/j.1471-0528.2010.02731.x.

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