Fertility Week 2020

November 2 marks World Fertility Day and the start of Fertility Week 2020.

Fertility Week aims to bring awareness and convey the issues faced by people struggling with infertility. To help raise awareness, the staff at Merrion Fertility Clinic have collaborated to provide information, insight and helpful tips on fertility through our website and social media platforms.

As part of this event, throughout the week we will be posting excerpts from Prof Mary Wingfield’s comprehensive guide to fertility: ‘The Fertility Handbook’ (Gill Press).

In the first of our Fertility Week multi-part series, Clinic Director Prof Mary Wingfield discusses the impact of obesity on fertility and pregnancy, and outlines how weight loss can benefit both.

Nutrition, Diet and Fertility

Adequate nutrition is essential for our general health and wellbeing. As a doctor, and also as a human being who is getting older, I am realizing more and more how a healthy diet and good nutrition can make us feel better and contribute to all aspects of our health and longevity. And fertility is no exception. In addition, medical problems related to obesity are now probably the biggest challenge we face in medicine.

There is no doubt that being overweight (BMI over 25) or obese (BMI over 30) increases the chances of infertility and miscarriage. Many obese women have no problem conceiving, but if we look at any study, they are proportionately a much smaller proportion of women of that weight than the proportion of women of normal weight who conceive. It’s a bit like smoking; we know that smokers die younger, but there will always be exceptions who smoke and live into their nineties.

Infertility has been shown to be almost three times higher in obese women, and obese adolescents have a greater chance of remaining childless than normal-weight women (1). While overweight women with polycystic ovarian syndrome will have irregular menstrual cycles, the adverse effect of weight on fertility is also seen in women with regular cycles (1). Some of these effects relate to increased oestrogen production in fat tissue, but there are also studies suggesting that egg quality is affected too.

Studies in women undergoing assisted reproduction treatments (ART) such as IVF and ICSI also show a negative effect of increased weight (2). Obese women have been reported to have a 68% lower chance of having a live birth following their first ART cycle compared with non-obese women; this seems to be related to poor ovarian response and lower rates of fertilisation, embryo transfer, implantation and pregnancy.

Blastocysts (embryos) developed from eggs of high-BMI women have been shown to be smaller, contain less cells, and have higher fat content, lower glucose consumption, and altered amino acid (protein) metabolism compared with embryos from normal-weight women (3). There is also evidence of an effect on the endometrium or womb lining. In addition, obesity is related to the requirement for increased doses of fertility drugs in treatment, more frequent cancellation of cycles and more difficult egg collection and embryo transfer procedures.

Miscarriage risk also increases with increasing BMI and has been shown to be one and a half to two times higher in obese and very obese women, both following natural pregnancy and following IVF/ICSI pregnancies (1). Even in women who have a successful pregnancy, pregnancy in overweight and obese women is associated with an increased risk of complications – for both the mother and the foetus – including diabetes, high blood pressure, pre-eclampsia, pre-term delivery, stillbirth, Caesarean or instrumental delivery, shoulder dystocia, foetal distress, early neonatal death and foetal abnormalities. They also risk having either abnormally small or large babies.

The benefits of weight loss

The good news is that weight loss in overweight and obese women (and less obviously so in men) has been shown to increase both natural conception rates and IVF success rates and also to improve the course of pregnancy (2).

This is particularly so for women who are overweight and have irregular ovulation (PCOS and non-PCOS): a reduction of 10% in weight leads to pregnancy in over 50% of couples. The classic study on this was published in Adelaide, Australia, in 1998 (4). Of 67 obese infertile women who were not ovulating and who lost an average of 10 kg each during a six-month weight-loss programme, 60 (90%) started ovulating and 52 (78%) conceived, with a miscarriage rate of 18%.

As regards couples undergoing fertility treatments like IVF, the news is also good.

A group in Sydney reviewed the literature and published a systematic review in 2014, looking at the effect of weight loss in overweight and/or obese women undergoing ART on their subsequent pregnancy outcome (2). Weight loss was significantly associated with increased pregnancy rates and/or live birth in eight of the 11 studies reviewed. In addition, regularisation of the menstrual pattern, a decrease in cancellation rates, an increase in the number of embryos available for transfer, a reduction in the number of ART cycles required to achieve pregnancy and a decrease in miscarriage rates were reported.


  1. ASRM Practice Committee (2015) ‘Obesity and reproduction: a committee opinion’. Fertility and Sterility 104: 1116–26.
  2. Sim, K. A., Partridge, S. R. and Sainsbury, A. (2014) ‘Does weight loss in overweight or obese women improve fertility treatment outcomes? A systematic review’. Obesity Reviews 15(10): 839–50.
  3. Leese, H. J. (2014) ‘Effective nutrition from conception to adulthood’. Human Fertility 17(4): 252–6.
  4. Clark, A. M., Thornley, B., Tomlinson, L. et al. (1998) ‘Weight loss in obese infertile women results in improvement in reproductive outcome for all forms of fertility treatment’. Human Reproduction 13(6): 1502–5.

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