Does body weight affect IVF outcomes?

This study looked at whether body weight in women affects the outcomes of in vitro fertilization (IVF). It found that women of normal weight had higher birth rates than women who were underweight or obese.

Weight affects many aspects of the body, including fertility. Women who are underweight may stop ovulating. Obesity can also cause long or irregular ovulation. In addition, bodyweight may affect other aspects of fertility such as the lining of the uterus. Body mass index (BMI) is an estimate of body weight in relation to height.

IVF is an infertility treatment in which the ovaries are stimulated, eggs are removed, and the fertilized embryos are implanted in the uterus. When multiple eggs are collected, embryos can be frozen for future cycles.

Studies have found that weight influences the success of IVF. However, most of these studies have studied a single implantation cycle. This does not account for embryos that are frozen for use in future cycles. It is unclear what effect bodyweight has on the cumulative birth rate from an IVF egg collection.

This study used records from 14,782 patients undergoing IVF. Some of the patients used intracytoplasmic sperm injection (ICSI), a modification of IVF for couples with male infertility. The study tracked all frozen embryos implanted from each egg collection. Couples were followed for two years. Because this study was done in Asia, it used lower cutoffs for body mass index (BMI) than are used with other populations.  

There was a significant difference in birth rate by body weight. Patients who were underweight (BMI under 18.5 kg/m²) had the highest birth rate of 66.41%. Patients with obesity (BMI of 28kg/m² or higher) had the lowest birth rate of 56.30%. 

The researchers used a statistical model to adjust for age, length of infertility, smoking, and other factors. After adjustment, patients with normal weight (BMI between 18.5 and 24 kg/m²) had the highest birth rates. Patients who were underweight had 14% lower odds of giving birth. Patients who were overweight (BMI between 24 and 28 kg/m²) also had 14% lower odds of birth compared to patients of normal weight. Patients with obesity had 37% lower odds of birth compared to patients of normal weight. Over a BMI of 30 kg/m², the birth rate fell sharply.

This study found that patients with normal BMI had the highest IVF birth rates, and patients with obesity had the lowest birth rates.

Bodyweight is connected to many other factors that were not measured, such as stress, sleep, income, and physical activity. It is difficult to disentangle these factors. Also, the study was done in Asian populations which are considered to have lower BMIs compared to non-Asian populations. This means that these results may not apply to all populations.