Reproductive Rounds

BMI and reproduction – weighing the evidence

Publish date: October 27, 2022

Arguably, no topic during an infertility consultation generates more of an emotional reaction than discussing body mass index (BMI), particularly when it is high. Patients have become increasingly sensitive to weight discussions with their physicians because of concerns about body shaming. Among patients with an elevated BMI, criticism on social media of health care professionals’ counseling and a preemptive presentation of “Don’t Weigh Me” cards have become popular responses. Despite the medical evidence on impaired reproduction with an abnormal BMI, patients are choosing to forgo the topic. Research has demonstrated “extensive evidence [of] strong weight bias” in a wide range of health staff.¹ A “viral” TikTok study revealed that medical “gaslighting” founded in weight stigma and bias is harmful, as reported on KevinMD.com.² This month, we review the effect of abnormal BMI, both high and low, on reproduction and pregnancy.

A method to assess relative weight was first described in 1832 as its ratio in kilograms divided by the square of the height in meters, or the Quetelet Index. The search for a functional assessment of relative body weight began after World War II when reports by actuaries noted the increased mortality of overweight policyholders. The relationship between weight and cardiovascular disease was further revealed in epidemiologic studies. The Quetelet Index became the BMI in 1972.³

Weight measurement is a mainstay in the assessment of a patient’s vital signs along with blood pressure, pulse rate, respiration rate, and temperature. Weight is vital to the calculation of medication dosage – for instance, administration of conscious sedative drugs, methotrexate, and gonadotropins. Some state boards of medicine, such as Florida, have a limitation on patient BMI at office-based surgery centers (40 kg/m²).

Obesity is a disease

As reported by the World Health Organization in 2022, the disease of obesity is an epidemic afflicting more than 1 billion people worldwide, or 1 in 8 individuals globally.⁴ The health implications of an elevated BMI include increased mortality, diabetes, heart disease, and stroke, physical limitations to activities of daily living, and complications affecting reproduction.

Female obesity is related to poorer outcomes in natural and assisted conception, including an increased risk of miscarriage. Compared with normal-weight women, those with obesity are three times more likely to have ovulatory dysfunction,⁵ infertility,⁶ a lower chance for conception,⁷ higher rate of miscarriage, and low birth weight.^8,9During pregnancy, women with obesity have three to four times higher rates of gestational diabetes and preeclampsia,¹⁰ as well as likelihood of delivering preterm,¹¹ having a fetus with macrosomia and birth defects, and a 1.3- to 2.1-times higher risk of stillbirth.¹²

Obesity is present in 40%-80% of women with polycystic ovary syndrome,¹³ the most common cause of ovulatory dysfunction from dysregulation of the hypothalamic-pituitary-ovarian axis. While PCOS is associated with reproductive and metabolic consequences, even in regularly ovulating women, increasing obesity appears to be associated with decreasing spontaneous pregnancy rates and increased time to pregnancy.¹⁴

Obesity and IVF

Women with obesity have reduced success with assisted reproductive technology, an increased number of canceled cycles, and poorer quality oocytes retrieved. A prospective cohort study of nearly 2,000 women reported that every 5 kg of body weight increase (from the patient’s baseline weight at age 18) was associated with a 5% increase in the mean duration of time required for conception (95% confidence interval, 3%-7%).¹⁵ Given that approximately 90% of these women had regular menstrual cycles, ovulatory dysfunction was not the suspected pathophysiology.

A meta-analysis of 21 cohort studies reported a lower likelihood of live birth following in vitro fertilization for women with obesity, compared with normal-weight women (risk ratio, 0.85; 95% CI, 0.82-0.87).¹⁶ A further subgroup analysis that evaluated only women with PCOS showed a reduction in the live birth rate following IVF for individuals with obesity, compared with normal-weight individuals (RR, 0.78; 95% CI, 0.74-0.82).

In a retrospective study of almost 500,000 fresh autologous IVF cycles, women with obesity had a 6% reduction in pregnancy rates and a 13% reduction in live birth rates, compared with normal-weight women. Both high and low BMI were associated with an increased risk of low birth weight and preterm delivery.¹⁷ The live birth rates per transfer for normal-weight and higher-weight women were 38% and 33%, respectively.

Contrarily, a randomized controlled trial showed that an intensive weight-reduction program resulted in a large weight loss but did not substantially affect live birth rates in women with obesity scheduled for IVF.¹⁸

Low BMI

A noteworthy cause of low BMI is functional hypothalamic amenorrhea (FHA), a disorder with low energy availability either from decreased caloric intake and/or excessive energy expenditure associated with eating disorders, excessive exercise, and stress. Consequently, a reduced GnRH drive results in a decreased pulse frequency and amplitude leading to low levels of follicle-stimulating hormone and luteinizing hormone, resulting in anovulation. Correction of lifestyle behaviors related to FHA can restore menstrual cycles. After normal weight is achieved, it appears unlikely that fertility is affected.¹⁹ In 47% of adolescent patients with anorexia, menses spontaneously returned within the first 12 months after admission, with an improved prognosis in secondary over primary amenorrhea.^20,21 Interestingly, mildly and significantly underweight infertile women have pregnancy and live birth rates similar to normal-weight patients after IVF treatment.²²

Pregnancy is complicated in underweight women, resulting in an increased risk of anemia, fetal growth retardation, and low birth weight, as well as preterm birth.²¹

Take-home message

The extremes of BMI both impair natural reproduction. Elevated BMI reduces success with IVF but rapid weight loss prior to IVF does not improve outcomes. A normal BMI is the goal for optimal reproductive and pregnancy health.

Dr. Trolice is director of the IVF Center in Winter Park, Fla., and professor of obstetrics and gynecology at the University of Central Florida, Orlando.