Being underweight can affect fertility, primarily by disrupting the hormonal signals that drive ovulation and sperm production. For women, the impact is most visible: a BMI below 18.5 is linked to irregular or absent periods, which directly reduces the chance of conceiving. For men, low body weight is associated with lower total sperm count and semen volume. The good news is that these effects are often reversible with weight gain and improved nutrition.
How Low Body Weight Disrupts Ovulation
Your body treats reproduction as a non-essential function when energy is scarce. When you’re underweight or in a calorie deficit, your brain reduces the pulsing release of a key reproductive hormone called GnRH. This hormone normally triggers the pituitary gland to release FSH and LH, the two hormones that stimulate your ovaries to mature and release an egg each month. Without adequate GnRH signaling, FSH and LH drop, and ovulation slows or stops entirely.
This condition is called functional hypothalamic amenorrhea, and it’s one of the most common causes of missed periods in underweight women. The word “functional” matters here: it means the reproductive system is structurally fine but has been switched to a low-power mode by the brain. Several metabolic signals drive this shutdown. When body fat is low, levels of leptin (a hormone produced by fat cells) fall. Insulin and a growth factor called IGF-1 also drop. Meanwhile, the stress hormone cortisol rises, and ghrelin, a hunger-signaling hormone, increases. All of these shifts converge on a single pathway in the brain involving a molecule called kisspeptin, which acts as the gatekeeper for GnRH. When kisspeptin signaling weakens, GnRH pulses become too infrequent for normal ovulation.
The result isn’t always a complete absence of periods. Some underweight women still menstruate but have cycles where no egg is actually released, or where the second half of the cycle is too short for an embryo to implant. These subtler disruptions can make conception difficult even when periods seem regular on the surface.
What the Numbers Show for Conception
The relationship between low BMI and fertility is real but more nuanced than many people assume. A large IVF study of nearly 15,000 patients found that underweight women (BMI below 18.5) had a cumulative live birth rate of 66.4%, compared to 65.8% for normal-weight women. After adjusting for age and other factors, the difference was not statistically significant. This suggests that once ovulation is restored through fertility treatment, being slightly underweight does not dramatically reduce the odds of a successful pregnancy.
That said, IVF bypasses the biggest barrier underweight women face: getting ovulation to happen in the first place. For women trying to conceive naturally, the picture is different. If low body weight has suppressed ovulation, no amount of timing or tracking will help until the underlying energy deficit is corrected. The study did note that gaining weight still improved outcomes, with roughly a 26% increase in cumulative live birth rate per unit of BMI gained.
Effects on Male Fertility
Underweight men face fertility challenges too, though the research is less extensive. A meta-analysis pooling data from multiple studies found that men with a BMI below 18.5 had significantly lower total sperm count and semen volume compared to normal-weight men. Sperm concentration (the density of sperm per milliliter) and motility (how well sperm swim) were not significantly different, which means the issue is more about producing fewer sperm overall rather than producing defective ones.
Researchers suspect the mechanism involves hormonal imbalances similar to what happens in women, though the specifics are less well mapped. Low energy availability can reduce testosterone production, which is the primary driver of sperm development. Nutritional deficiencies common in underweight individuals, particularly zinc and vitamin D, may also play a role in reduced sperm output.
Nutrient Gaps That Compound the Problem
Being underweight often goes hand in hand with nutritional deficiencies, and several key nutrients directly support reproductive function. Iron deficiency can impair ovulation. Low folate (vitamin B9) affects DNA synthesis in rapidly dividing cells, including eggs and early embryos. Vitamin D deficiency, common in people who undereat or restrict food groups, is linked to poorer outcomes in both natural conception and assisted reproduction. Zinc, selenium, and magnesium all play supporting roles in egg and sperm quality.
These deficiencies can quietly worsen fertility even when someone’s weight is only slightly below normal. A person who maintains a BMI of 18 through a nutritionally balanced diet may face fewer reproductive consequences than someone at the same weight who restricts entire food groups. The total picture of energy and nutrient availability matters more than the number on the scale alone.
Pregnancy Risks for Underweight Women
Fertility isn’t just about getting pregnant. It’s also about carrying a healthy pregnancy, and being underweight at conception raises specific risks. According to the U.S. Office on Women’s Health, babies born to mothers with a BMI below 18.5 face higher rates of premature birth (delivery before 37 weeks) and low birth weight (under 5.5 pounds). Low birth weight infants are more likely to have health and developmental challenges as they grow.
These risks mean that even if you conceive successfully while underweight, gaining weight before or during early pregnancy can improve outcomes for both you and the baby. Most of the risk comes from inadequate nutrient stores and reduced blood volume, both of which improve with better nutrition and gradual weight gain.
How Recovery Works
The most encouraging aspect of underweight-related infertility is its reversibility. Functional hypothalamic amenorrhea resolves in most women once energy balance is restored. This typically means eating more calories than you burn, consistently, over weeks to months. Periods often return once the body senses that energy stores are sufficient to support a pregnancy.
There’s no single BMI threshold that guarantees the return of ovulation. Some women resume cycling at a BMI of 19, while others need to reach 21 or higher. The key factor isn’t a specific weight but rather the resolution of energy deficit. Women who are underweight due to high exercise volume sometimes need to reduce training intensity alongside increasing food intake. Stress reduction also helps, since cortisol independently suppresses GnRH signaling.
For men, improving caloric intake and correcting nutritional gaps can increase sperm production over the course of about three months, which is the time it takes for a new cycle of sperm to fully mature. Weight gain doesn’t need to be dramatic. Even modest increases that move BMI into the normal range can make a measurable difference in total sperm count and semen volume.