Obese couple
Obese couples take longer to conceive according to the study Istock

Couples in which both partners are obese may take longer to conceive, scientists have found out. The data indicates that they may take from 55 to 59% longer get pregnant compared to their leaner counterparts.

A number of studies have investigated the role of women's weight on their fertility, showing that obesity may have an adverse effect. It is also known that obesity is associated with less successful fertility treatments.

However, despite pregnancy being a couple-dependant outcome, this research is the first to look at the link between body mass index (BMI) in both members of a couple and the time it takes them to get pregnant.

Published in the journal Human Reproduction, the study focused on the case of 501 couples, recruited from 16 counties in Michigan and Texas between 2005 and 2009.

All the participants had their BMI measured as well as their waist and hip circumferences. They were divided into groups - obese and non-obese. A total of 75 men and 69 women were considered to be obese. They were in turn organised in two subgroups - obese class I (with a BMI from 30 to 34.9), and the more obese class II (a BMI of 35 or greater).

All couples were followed up daily for a year, or less, if they got pregnant before that. Women were asked to keep journals to record their monthly menstrual cycles, intercourse and the results of home pregnancy tests.

Using this data, the scientists, from NIH's Eunice Kennedy Shriver National Institute of Child Health and Human Development, calculated the couples' fecundability odds ratio (FOR). For couples who are trying to conceive, this measures their probability of getting pregnant during each menstrual cycle relative to their BMIs.

Longer time to achieve pregnancy

Having only one obese member in the couple did not significantly alter the time they had to wait to conceive.

The picture was however different when both partners were obese. Obese couples had lower FORs compared with non-obese couples, and took 55% longer to achieve pregnancy than average-sized couples.

When controlling for other factors that can influence fertility, such as smoking, age or cholesterol level, the researchers found that the effect of a high BMI was even greater — obese couples took 59% longer to achieve pregnancy.

This shows that the BMI of both partners can have an impact on their chances of becoming parents. "Our results also indicate that fertility specialists may want to consider couples' body compositions when counselling patients," senior author Rajeshwari Sundaram said.

However, the scientists recognise that more research may be needed, as their findings may not apply to the whole population. BMI and waist circumference are indeed proxy measures of body composition and may not always adequately describe obesity.

Commenting on the study, Professor Adam Balen, Chair of the British Fertility Society said: "This study looks at a relatively small population but highlights the important issue of a healthy body weight for both men and women. It looks at couples, when other studies have focused more on maternal obesity only, and provides insight into the effect for both men and women who may well share the same lifestyle and habits.

"The prevalence of obesity has increased over the years and having a normal body weight increases the chances of conceiving naturally and reduces the risk of complications for both mother and baby. It is recommended that men and women actively maintain a healthy weight, exercise and eat healthily prior to starting a family."