Pre-pregnancy body mass index and time to pregnancy among couples pregnant within a year: A China cohort study.

Yuhang Fang,Min Li,Yanyan Mao,Liu Yang,Yang He,Qi Tong,Weijin Zhou,Qianxi Zhu,Jun Liu,Frank T Spradley

doi:10.1371/journal.pone.0231751

Abstract

BackgroundExtreme pre-pregnancy body mass index (BMI) values have been associated with reduced fecundability and prolonged time to pregnancy in previous studies. However, the effect in fertile couples is unclear.ObjectivesThis study aimed to evaluate the association between pre-pregnancy BMI and fecundability, measured as time to pregnancy (TTP), among couples that achieved pregnancy within 1 year.MethodsThis was a retrospective cohort study of 50,927 couples wishing to conceive, enrolled in the National Free Preconception Health Examination Project (NFPHEP) in Chongqing, China, during 2012–2016. Participants’ weight and height were measured by NFPHEP-trained preconception guidance physicians. TTP measured in months was used to determine subfecundity (TTP >6 months). The strength of association between BMI and TTP/subfecundity was measured with fecundability odds ratios (FOR)/odds ratios (OR) and their corresponding 95% confidence intervals (CI), calculated with Cox and logistic regression analysis. We used restricted cubic spline regression (RCS) to test the observed FOR trends.ResultsCompared to women with normal BMI, women with pre-pregnancy overweight/obesity had longer TTP (FOR = 0.96, 95% CI: 0.94–0.99) and increased risk of subfecundity (OR = 1.08, 95% CI: 1.00–1.17). There was no association between TTP and male BMI. RCS trends varied when data were stratified by male pre-pregnancy BMI, with the greatest change detected in pre-pregnancy underweight men.ConclusionsPre-pregnancy overweight/obesity was associated with longer TTP and subfecundity among women who became pregnant within 1 year; this effect was likely mediated by their partners’ pre-pregnancy BMI. These findings indicate that BMI could affect fecundability, independently of affecting the risk of sterility. Advice on weight management and maintaining healthy weight should be included in couples’ preconception guidance.

Highlights

Obesity affects 13% of the world population and 12.4% of adults in China [1, 2]
Compared to women with normal body mass index (BMI), women with pre-pregnancy overweight/obesity had longer time to pregnancy (TTP) (FOR = 0.96, 95% confidence intervals (CI): 0.94–0.99) and increased risk of subfecundity (OR = 1.08, 95% CI: 1.00–1.17)
There was no association between TTP and male BMI

Summary

Introduction

Obesity affects 13% of the world population and 12.4% of adults in China [1, 2]. Previous research has suggested that obesity might damage couples’ reproductive capacity through several biological processes [3,4,5,6], such as hypogonadism and abnormal germ cell production. Epidemiologists have used body mass index (BMI) [11] and time to pregnancy (TTP) [12] to assess the relationship between extreme body weight and fecundability at a population level. Some previous studies have suggested that lower fecundability, reflected in prolonged TTP, was associated with higher BMI or higher body fat percentage [11, 13,14,15,16,17,18,19,20,21,22,23,24], but these findings are inconsistent [25, 26]. No previous study has evaluated the relationship between BMI and TTP in fertile populations (TTP 12 months). Extreme pre-pregnancy body mass index (BMI) values have been associated with reduced fecundability and prolonged time to pregnancy in previous studies.

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