Both partners’ BMI in couples hoping to conceive has an effect on time to pregnancy


While evidence is strong that high BMI has an adverse effect on female fertility, little is known of how ‘abnormal’ pre-pregnancy BMIs in women and men are associated with time to pregnancy. A large cohort study from China suggests that management of couples’ pre-pregnancy BMI might improve their fertility.

While the effect of bodyweight on conception has long been a subject of study, those studies have tended to concentrate on excess weight, on the female partner and on assisted reproduction. In ART, for example, there is evidence that a high BMI raises the risk of fewer oocytes, poorer egg quality, reduced fertilisation rates, and lower implantation rates – as well as pregnancy and obstetric complications.(1) In some countries, and in some clinics, the evidence is deemed sufficiently strong to make BMI an eligibility criterion for treatment.

Now, a nationwide cohort study based on couples attending preconception clinics in China has found a prolonged time to pregnancy (used in this study as an index of fertility) when both the female and male partners
had abnormal BMIs (as reflected in both under and over weight).(2) The couples in the study, which lasted throughout 2016 and 2017, totalled more than 2 million and all were planning their first pregnancy. While some of the baseline data was self-reported, the couples’ blood pressure, height and weight were professionally measured, and after these examinations the couples were contacted by telephone every three months to check on pregnancy progress. Both male and female results were adjusted for well recognised variables.

For the women, the pregnancy rates of the underweight, normal BMI, overweight, and obese groups were 61.62%, 64.14%, 61.50%, and 55.01%, respectively; and for the men 62.74%, 63.32%, 63.72%, and 62.22% at the end of a one-year follow-up period. However, after adjustments the women who were underweight, overweight and obese had a 7.00%, 5.00% and 20.00% lower possibility of pregnancy than those who had a normal BMI. There was a similar finding among underweight men, but the results were inconsistent in the overweight and obese men.

The results from the couple-based models showed that, when compared with couples where both partners had a normal BMI, pregnancy rates were reduced by 10% and 19% in those where both partners were either underweight or obese – although there was no statistically significant difference among couples in whom both partners were overweight. Based on time-to-pregnancy, the study found that a combination of normal-BMI women and overweight men had the greatest fertility, and a combination of obese women and underweight men had the lowest fertility. Optimal BMI for the shortest time to pregnancy in the cohort were noted as 20.61-23.06 and 22.69-27.7 for females and males respectively.

The investigators, who described this as ‘the largest prospective study in the world on couples’ pre-pregnancy BMI and fecundability’, conclude that the pre-pregnancy examination has important public health consequences, and that ‘the scientific management of prepregnancy BMI levels of female and male partners may improve the couple’s fertility’. This is a similar conclusion to that raised in an editorial accompanying the report, in which the study’s findings are described as ‘clinically applicable’ to couples considering pregnancy.(3) This is especially so today, when BMI levels continue to rise as a result of diet and increasingly sedentary lifestyles, and when more and more couples are turning to IVF. ‘Counseling on optimizing preconception health, particularly for those in the preconception time window must be thoughtfully addressed within the reproductive endocrinology visit’, the editorial concludes.

Meanwhile, such recommendations have been put to an everyday test – for women and not couples, however – in an Australian study in which women with a high BMI (30-55) planning conception were randomised to a standard weight loss diet for 12 weeks or a ‘very low energy diet’.(4) Weight loss at the end of the trial was 3.1% in the standard diet group and 11.9% in the very low energy diet group. Similarly, time to pregnancy was significantly shorter in the latter group than in the former. Further analysis showed that this difference in time to conception was particularly evident within 90 days of the intervention. Again, the authors conclude – as reflected in current guidelines – that a short-duration weight loss programme in obese women resulting in significant preconception weight loss may result in a shorter time to conception than if conception is attempted without weight loss. Moreover, they add that the pre-pregnancy low energy diet programme ‘largely overcame the delay in time to pregnancy resulting from obesity’. 

1 Luke B, Brown MB, Missmer SA, et al. The effect of increasing obesity on the response to and outcome of assisted reproductive technology: a national study. Fertil Steril 2011; 96: 820–825; https://doi.org/10.1016/j.fertnstert.2011.07.1100
2. Zhang Y, Zhang J, Zhao J, et al. Couples’ pre- pregnancy body mass index and time to pregnancy among those attempting to conceive their first pregnancy. Fertil Steril 2020. doi.org/10.1016/j.fertnstert.2020.05.041
3. Mahalingaiah S. It’s time to include couple-based body mass index counseling in the infertility clinic visit. Fertil Steril 2020; doi.org/10.1016/j.fertnstert.2020.07.058 
4. Price SA, Sumithran P, Prendergast LA, et al. Time to pregnancy after a prepregnancy very-low-energy diet program in women with obesity: substudy of a randomized
controlled trial. Fertil Steril 2020; doi.org/10.1016/j.fertnstert.2020.06.033

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