BMI of both partners affects time to pregnancy

Optimal body mass index (BMI) of both female and male partner decreases time to conception in first-time pregnancies, a large-scale study has shown.

A correlation between high BMI in women and lower fertilisation rates – as well as more pregnancy complications – has long been established. However, how the bodyweight of both partners affects how long it takes to establish a pregnancy has not been previously investigated. 

'This was the largest prospective study in the world on couples' pre-pregnancy BMI. The scientific management of pre-pregnancy BMI levels of female and male partners may improve the couple's fertility,' wrote the authors of the study, which was published in the journal Fertility and Sterility. 

Researchers from the National Research Institute for Family Planning in Beijing, China, recorded the BMIs of two million couples trying for a first child over a two-year period. The individuals were assigned to four bands based on BMI: underweight, normal, overweight and obese. Time to pregnancy was used as an index of fertility.

Compared to couples where both were in the normal BMI range, reproduction rate was decreased by ten percent when both partners were underweight or 19 percent when both were obese. There was no statistically significant difference between couples with normal BMIs and those who were classified as overweight. 

Overall women who were underweight, overweight or obese were seven percent, five percent and 20 percent respectively less likely to conceive than those who had a normal BMI. Underweight men were also less likely to have a successful conception, but the results for overweight and obese men were less conclusive.

The study's authors cautioned that their results did not prove the relationship between BMI and time to pregnancy was causal, and warned that they may not be generalisable as the subjects 'were limited to younger couples attempting to conceive their first pregnancy, who were healthier than the general population.'

'It is time to include couple-based body mass index counselling in the infertility clinic visit', said reproductive and women's health researcher Dr Shruthi Mahalingaiah, from the Harvard School of Public Health in Boston, Massachusetts, who was not involved in the study. 'It is critically important to consider and optimise health factors in the population at large and at a couple (male plus female) level to not only reduce the risk for infertility but also to improve gestational and birth outcomes.'