- Low BMI below 18.5 can cause hormonal imbalances and ovulation issues in women
- Functional Hypothalamic Amenorrhea occurs when low weight disrupts menstrual cycles
- Leptin hormone from fat signals energy sufficiency for reproduction and ovulation
When conversations around fertility and weight arise, the spotlight often falls on obesity. However, doctors are increasingly warning that being underweight can be just as disruptive, sometimes even more silently damaging, to reproductive health. Women with a Body Mass Index (BMI) under 18.5 may face hormonal imbalances, irregular ovulation, and higher pregnancy risks. According to the World Health Organization (WHO), a BMI below 18.5 is classified as underweight. While the public health narrative often focuses on metabolic diseases linked to high BMI, evidence shows that inadequate body fat can interfere with the delicate hormonal systems necessary for conception.
Dr. Anupama Ashok, Senior Consultant and Fertility Expert at Motherhood Fertility & IVF, Hebbal, Bengaluru, explains that the body requires a minimum energy reserve to prioritise reproduction. "The human body requires a particular energy level which acts as a basic requirement for achieving reproductive health," she says. "When body fat drops below a critical threshold, the brain interprets it as a state of energy crisis."
How Low Body Weight Disrupts Ovulation
At the centre of reproductive regulation is the hypothalamus, a small but powerful gland in the brain. It controls hunger, temperature, and crucial reproductive hormones. "To comprehend the effects of low body weight on reproductive success, it is necessary to study how the brain functions as the body's energy management system," says Dr. Ashok. "When a person's body weight or fat percentage drops below a critical threshold, the hypothalamus detects it as a permanent energy shortage."
This can trigger a condition known as Functional Hypothalamic Amenorrhea (FHA), in which menstrual periods become irregular or stop altogether. Research published by the National Institutes of Health (NIH) shows that FHA accounts for up to 30% of secondary amenorrhea cases in women of reproductive age.
"In FHA, the hormones that tell your ovaries to release an egg slow down because the pituitary gland does not receive the chemical messengers required for egg maturation and ovulation initiation," Dr. Ashok explains.
The Role Of Leptin: The "Green Light" Hormone
One of the key hormonal players is leptin, a hormone produced by fat cells. "The secret messenger here is a hormone called leptin," says Dr. Ashok. "Produced by body fat, leptin acts like a green light for the reproductive system. When you don't have enough body fat, leptin levels plummet, and the green light turns red. It tells your brain, 'We do not have enough stored energy to support a baby.'"
Studies have shown that low leptin levels are directly associated with disrupted ovulation and infertility. This is why athletes, ballet dancers, and women on highly restrictive diets often experience missing periods.
Pregnancy Risks Linked To Low BMI
Even if conception occurs, being underweight may increase pregnancy complications. Research supported by the Indian Council of Medical Research (ICMR) has also highlighted the strong link between maternal undernutrition and poor neonatal outcomes in India. According to WHO data and global maternal health studies, low pre-pregnancy BMI is associated with:
- Higher risk of preterm birth
- Babies born small for gestational age (SGA)
- Increased risk of low birth weight
"The impact doesn't end with a positive pregnancy test," Dr. Ashok notes. "Low pre-pregnancy weight is linked to higher risks of preterm birth and babies that are small for their age because they didn't have a deep enough well of maternal nutrients to draw from."
It Affects Men Too
While fertility discussions often centre on women, low body weight in men can also impair reproductive health. "Low body weight in males is often tied to lower testosterone and lower sperm counts," says Dr. Ashok. "The body prioritises basic survival over making healthy sperm."
Finding The "Fertility Zone"
Importantly, improving fertility is not about crash weight gain. Instead, doctors advocate reaching what Dr. Ashok calls a "Fertility Zone." "Getting reproductive success is not about consuming junk food to get a number higher on the scale; it is about turning your lifestyle around," she says. "Eat nutrient-rich foods such as healthy fats from avocados, nuts, and oils, and consider switching from high-intensity, stressful cardio to strength-building activity."
Balanced nutrition, adequate protein intake, iron sufficiency, and healthy fats play key roles in restoring menstrual regularity. The WHO recommends that women planning pregnancy maintain a healthy BMI between 18.5 and 24.9 for optimal outcomes.
Also Read: Can Using Birth Control Pills Cause Infertility? Fertility Doctor Answers Common Query Women Have
Shifting The Narrative Around Thinness
Modern beauty standards often glorify thinness. However, reproductive health depends on abundance, adequate nourishment, hormonal balance, and metabolic security. "At the end of the day, fertility is a sign of abundance," says Dr. Ashok. "It is the body's way of telling us that it feels secure, well-nourished, and strong enough to give life."
By shifting the focus from achieving a lower number on the scale to achieving metabolic and hormonal balance, couples may significantly improve their chances of conception.
While obesity-related fertility risks deserve attention, being underweight is an equally important but often overlooked cause of infertility. Low BMI can disrupt ovulation, lower sperm quality, and increase pregnancy complications. Reproductive success depends on adequate energy reserves, hormonal harmony, and balanced nutrition.
For individuals struggling with unexplained infertility, assessing BMI and nutritional health may be a critical first step. In fertility, as science increasingly confirms, the body must feel nourished and safe before it is ready to create new life.
Disclaimer: This content, including advice, provides generic information only. It is in no way a substitute for a qualified medical opinion. Always consult a specialist or your own doctor for more information. NDTV does not claim responsibility for this information.
