SEVERE THINNESS AND LIVE BIRTH OUTCOME FOLLOWING IN VITRO FERTILIZATION

Purpose: An underweight body mass index (BMI) has been linked to decreased likelihood of a live birth outcome for women undergoing in vitro fertilization (IVF). However, the World Health Organization has subcategorized underweight BMI into severe, moderate, and mild thinness to facilitate international comparisons of BMI, and less is known about these subcategories and IVF outcomes. Using data from a university-affiliated infertility practice, this study evaluates the relationship between severe thinness, a subcategory of underweight, and the likelihood of a live birth outcome for women undergoing IVF.

Method: This retrospective study was performed on 1629 women undergoing first autologous fresh IVF cycle at the center between 2001 and 2008. The primary outcome was live birth. The patients were classified into five groups according to BMI category: <16.00kg/m² (severe thinness); 16.00-18.50kg/m² (moderate/mild thinness); 18.50-24.99 kg/m² (normal); 25.00-29.99kg/m² (overweight); 30.00 kg/m²(obese). Analyses controlled for potential confounders, including insurance coverage for IVF services, age, total drug, peak estradiol level, number of retrieved oocytes, and number of embryos transferred.

Result: A multiple logistic regression was run to predict the odds of a live birth outcome following IVF. The first IVF cycle of 1070 patients was analyzed. The model showed that compared with all other weight categories, severe thinness significantly decreased the odds of a live birth outcome after controlling for potential confounders. Moderate/mild thinness (OR=1.14, 95% CI = .47-2.80), normal weight (OR=1.94, 95% CI = 1.20-3.15), overweight (OR=1.82, 95% CI = 1.08-3.06), and obese females (OR=1.75, 95% CI = 1.03-2.98) were 14%, 94%, 82%, and 75% more likely to have a live birth outcome than severely thin women, respectively. Maternal age, number of oocytes retrieved, and number of embryos transferred were also important factors. 

Conclusion: These findings demonstrate that the odds of a live birth is dramatically decreased in women who are severely thin, compared with women of all other weight categories. While the moderate/mild thinness BMI category was not statistically significant, this can be attributed to statistical power issues because the majority of underweight women in the sample were severely thin. Evidence can be used to educate patients about the beneficial lifestyle changes and appropriate weight gain required to optimize IVF treatment.