Body Mass Index (BMI)

Body mass index (BMI) is a measure that compares body weight with height. BMI equals weight in kilograms divided by height in meters squared (kg/m²). It is a screening tool for weight-related health risk across large populations.

The World Health Organization sets the ranges. A BMI below 18.5 is underweight. A BMI of 18.5 to 24.9 is normal, and 25.0 to 29.9 is overweight. A BMI of 30.0 or above is obese. BMI does not measure body fat or metabolic health directly. Its limitations for any individual person are well known.^[69]

A BMI at either extreme impairs fertility. The risk of reduced fertility (subfertility) rises as BMI moves further from the normal range, in both directions.^[69]

At the low end, the body does not receive enough energy. This energy deficit drives the loss of periods (hypothalamic amenorrhea) and the absence of ovulation (anovulation).

At the high end, the body responds poorly to insulin (insulin resistance), and insulin levels rise. This process drives cycles without ovulation and an excess of androgens, the male-type hormones. It is the core metabolic problem in many cases of polyendocrine metabolic ovarian syndrome (PMOS), formerly polycystic ovary syndrome (PCOS).

Obesity is also linked to a higher risk of miscarriage and to lower pregnancy rates per cycle. The cause is metabolic. Body fat (adipose tissue) acts as a gland that produces hormones.

Adipose tissue produces excess estrogen. It disrupts the signals between the brain and the ovaries (the hypothalamic-pituitary-ovarian axis). It also worsens insulin resistance.

Correction of the underlying metabolic problem through changes in diet and daily habits treats the root cause instead of bypassing it.^[70]

When a couple has unexplained infertility, clinicians check BMI. They interpret it alongside any hormonal abnormalities. BMI helps explain the findings on the cycle chart. It also helps guide changes to diet and daily habits. The goal is to restore normal ovulation, not only to reach a target weight.