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This Cochrane review finds that the risk of over-stimulating the ovaries with gonadotrophin-releasing hormone (GnRH) antagonists is between 6% and 9% compared to about 11% with GnRH agonists. Both treatments resulted in similar live birth rates of around 29% for women undergoing fertility treatment.

GnRH agonists replace the body’s natural hormone cycle to provide a steady rise in levels of the hormones that trigger egg-release, preventing hormonal surges that may cause the treatment to fail. GnRH antagonists are increasingly being used in the UK but work in a different way. They block the natural hormones at a key point in the treatment cycle to prevent hormone surge.

The findings reinforce 2013 NICE guidelines to choose a GnRH antagonist if the woman is at risk of ovarian hyper-stimulation. Cost-effectiveness was not assessed and this would need to be factored into decision-making if considering their wider use.

Why was this study needed?

In 2013, nearly 50,000 women in the UK who had difficulties conceiving a baby naturally received “assistive reproductive technology” commonly known as fertility treatment, with 2.2% of all babies born in the UK conceived this way.

Some form of fertility treatment use drugs and artificial hormones to stimulate the woman’s body to produce eggs, which are collected and fertilised externally in the laboratory, then re-implanted into the woman. The two most commonly used forms of fertility treatment are in vitro fertilisation – where the egg is mixed with multiple sperm – and intra-cytoplasmic sperm injection – where a single sperm is injected directly into the egg to fertilise it.

Prior to egg collection, the process has to be carefully managed to ensure that the body’s hormones are perfectly balanced and maximise the chances of success.

Ovarian hyperstimulation syndrome is a potentially serious complication of hormone treatment for infertility. It can cause nausea, vomiting, dehydration and, rarely, death.

This review compared two methods of controlling the action of the hormone gonadotrophin to see which method would result in the largest number of live births and the lowest rate of adverse effects.

What did this study do?

This systematic review identified 73 randomised controlled trials including a total of 12,212 women undergoing fertility treatment. Studies compared the effectiveness of either GnRH agonists or GnRH antagonists. Where possible, studies were pooled in meta-analysis to compare their effects on the rate of live births and the adverse effect of over-stimulating the ovaries.

The included studies for the main outcomes were judged as moderate quality partly because the risk of bias was unclear from the study reports. Some studies had a risk of bias because of the way that patients were allocated to the treatment group.

What did it find?

  • There was no difference in the rate of live births following either GnRH antagonist or GnRH agonists (odds ratio [OR] 1.02, 95% confidence interval [CI] 0.85 to 1.23; meta-analysis of 12 studies). The live birth rate was about 29% with a GnRH agonist and 25 to 33% after a GnRH antagonist.
  • GnRH antagonists significantly reduced the risk of ovarian hyperstimulation syndrome compared to GnRH agonists (OR 0.61, 95% CI 0.51 to 0.72; 36 studies), with a rate of about 6-9% with an antagonist compared with 11% with an agonist.
  • Fewer cycles were cancelled due to the risk of ovarian hyperstimulation syndrome in the GnRH antagonist group (OR 0.47, 95% CI 0.32 to 0.69; 19 studies). However, more cycles were also cancelled in this group due to poor ovarian response (OR 1.32, 95% CI 1.06 to 1.65; 25 studies).
  • There was no difference between GnRH antagonist and GnRH agonists in the number of on-going pregnancies (lasting beyond 12 weeks) or the rate of miscarriages.

What does current guidance say on this issue?

NICE 2013 guidelines recommend using a long course of GnRH agonists in women who are at a low risk of ovarian hyperstimulation syndrome. Women with an increased risk should be offered GnRH antagonists.

What are the implications?

This review indicates that GnRH antagonists are just as likely as GnRH agonists to result in live birth, and may be safer in terms of reducing the risk of over-stimulating the ovaries.

This is a large body of evidence, and the findings reinforce NICE recommendations that women at increased risk of ovarian hyperstimulation syndrome should be prescribed antagonists.

This review did not include an economic evaluation, so it is unclear whether there is any difference in the cost-effectiveness of either treatment.

Citation and Funding

Al-Inany HG, Youssef MA, Ayeleke RO,et al. Gonadotrophin-releasing hormone antagonists for assisted reproductive technology. Cochrane Database Syst Rev. 2016;4:CD001750.

No funding information was provided for this study.


HFEA. Fertility treatment in 2013:trends and figures. London: Human Fertilisation & Embryology Authority; 2014.

HFEA. IVF - What is in vitro fertilisation (IVF) and how does it work? London: Human Fertilisation & Embryology Authority; updated 2014.

HFEA. What is intra-cytoplasmic sperm injection (ICSI) and how does it work? London: Human Fertilisation & Embryology Authority; updated 2015.

NICE. Fertility problems: assessment and treatment. CG156. London: National Institute for Health and Care Excellence; 2013.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre

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When the ovaries are stimulated to produce eggs for collection, there is a risk of a surge of lutenising hormone that results in the egg being released before it was the optimal size and prevents the collection of multiple eggs, meaning that the assistive reproductive technology cycle fails.

This lutenising hormone surge can be controlled using artificial gonadotrophin-releasing hormone (GnRH) agonists which occupy the receptors of natural gonadatrophin and lead to a more controlled rate of hormone rise throughout the process. However, GnRH agonists have been associated with ovarian hyperstimulation syndrome which can be dangerous as it leads to fluid build-up in the abdomen and can lead to blood clots.

GnRH antagonists may avoid this complication, by blocking the natural gonadotrophin at a key point in the process where there is an increased risk of lutenising hormone surge.

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