Can Being Underweight Affect Fertility? What Science Says
Being underweight can disrupt hormones and ovulation, making pregnancy harder. Learn how low BMI affects fertility and when to seek professional help.
Quick answer: Yes. Being underweight can affect fertility, especially when low weight is linked to too little food, recent weight loss, intense exercise, an eating disorder or illness. A shortage of available energy can disrupt the hormones that control ovulation. Periods may become irregular or stop, which can make pregnancy harder. However, a low body mass index (BMI) does not automatically mean you are infertile. Some naturally slim people continue to ovulate and become pregnant without treatment.
How Does Being Underweight Affect Fertility?
In women, the main effect is usually irregular or absent ovulation.
For a detailed explanation of menstrual changes, read how being underweight can affect periods. The brain, pituitary gland and ovaries work together to control the menstrual cycle. This system is called the hypothalamic-pituitary-ovarian axis. When the body does not receive enough energy, the brain may reduce the signals that support reproduction.
This can cause:
- Changes in gonadotropin-releasing hormone, luteinising hormone and follicle-stimulating hormone
- Lower oestrogen levels
- Irregular follicle development
- Late, irregular or absent ovulation
- Irregular periods or no periods
- Fewer chances to conceive during the year
When periods stop because of low energy intake, weight loss, heavy exercise or stress, doctors may diagnose functional hypothalamic amenorrhoea. It is a diagnosis of exclusion, which means pregnancy and other medical causes must be ruled out first.[1]
Low BMI Is Not the Same as Low Energy Availability
The World Health Organization classifies a BMI below 18.5 kg/m² as underweight in adults.[2] BMI is useful for screening, but it is not a fertility test.
BMI does not directly measure:
- Food or nutrient intake
- Recent weight loss
- Body fat distribution
- Hormone levels
- Ovulation
- Sperm quality
- The amount of energy left after exercise
Low energy availability means the body does not have enough energy to support all its normal functions after accounting for physical activity. It can happen because a person eats too little, exercises more than their intake can support or has an illness that affects appetite or nutrient absorption.
This distinction matters. A naturally slim person with stable weight, adequate nutrition and regular cycles may have no weight-related fertility problem. Another person can develop irregular ovulation after rapid weight loss or under-fuelling even if their BMI remains above 18.5. Research in athletes also shows that low energy availability can affect reproductive function in both women and men.[3]
Calculate Your BMI with our Advanced BMI Calculator.
Can You Get Pregnant If You Are Underweight?
Yes. Pregnancy is possible if ovulation occurs and there are no other fertility problems. Being underweight does not work like contraception.
Regular menstrual cycles lasting about 21 to 35 days are generally a reassuring sign that ovulation is occurring. The American Society for Reproductive Medicine states that extra testing to confirm ovulation is usually unnecessary in women with regular 21-to-35-day cycles unless another concern, such as hirsutism, is present.[4]
However, periods do not confirm that every part of the reproductive system is healthy. Pregnancy also depends on age, the fallopian tubes, the uterus, timing of sex and sperm health.
If you are underweight with regular periods: Your fertility may not be affected, particularly if your weight is stable and you eat enough. Seek an assessment if pregnancy does not occur within the recommended time.
If you are underweight with irregular or absent periods: Ovulation may be happening less often or may have stopped. This gives you fewer chances to conceive and is a reason to seek medical advice sooner.
A 2024 prospective cohort study found that underweight women had higher odds of subfertility than women in the normal BMI range. However, this was an observational association. It does not mean that every underweight woman will have difficulty becoming pregnant.[5]
What BMI Is Too Low to Get Pregnant?
There is no exact BMI at which pregnancy becomes impossible.
A BMI below 18.5 is classified as underweight, but it does not prove that ovulation has stopped. Some people become pregnant with a BMI below this level, while others develop cycle problems at a higher BMI after rapid weight loss or prolonged under-eating.
BMI is more concerning when it occurs with:
- Irregular or stopped periods
- Recent or unplanned weight loss
- Restricted eating
- Intense exercise without enough food
- Nutrient deficiency
- An eating disorder
- A medical condition causing low weight
Your menstrual pattern, weight history, food intake, exercise and overall health are more informative than one BMI number.
Signs Low Weight May Be Affecting Fertility
Possible warning signs include:
- Periods becoming less regular
- Cycles becoming much longer
- Missing periods
- Trouble detecting ovulation
- Recent or rapid weight loss
- Constant tiredness or feeling cold
- Low sexual desire
- Poor recovery from exercise
- Frequent injuries or stress fractures
- Restrictive eating or fear of weight gain
- Compulsive exercise
These signs do not prove that low weight is the cause. Thyroid disease, polycystic ovary syndrome, high prolactin, premature ovarian insufficiency, anaemia, digestive conditions and other illnesses can cause similar symptoms.
Why Is the Person Underweight?
The cause of low weight often matters more than the number on the scale.
Possible causes include:
- Eating too little overall, even when the foods chosen are nutritious
- Restrictive dieting or skipped meals
- Rapid weight loss
- Intense exercise without enough food and recovery
- An eating disorder or disordered eating
- An overactive thyroid
- Coeliac disease, inflammatory bowel disease or poor nutrient absorption
- Uncontrolled diabetes or another chronic illness
- Depression, anxiety, low appetite or medication side effects
If low appetite or early fullness is limiting food intake, learn how to increase appetite naturally for weight gain. Do not assume that an underweight person only needs to “eat more.” Unexplained weight loss should be medically assessed so the underlying cause can be treated.
Can Gaining Weight Improve Fertility?
It can, particularly when low energy availability has disrupted hormone signals and ovulation.
The Endocrine Society recommends correcting the energy imbalance in people with functional hypothalamic amenorrhoea. This may involve eating more, improving nutrition, reducing excessive exercise or combining these changes. Weight gain is often part of recovery.[1]
There is no single amount of weight that restores fertility for everyone. Reaching a BMI of 18.5 does not guarantee that periods or ovulation will return. Some people may need to move closer to the weight at which they previously had regular cycles.
If weight gain is appropriate, learn how many calories you should eat to gain weight gradually. People with severe underweight, an eating disorder or another medical condition should use a clinician-led plan.
Recovery time also varies. Periods may return within a few months for some people but take longer for others. A doctor and registered dietitian can set a safe target based on usual weight, recent weight loss, activity, symptoms and medical history.
Anyone with an eating disorder, severe underweight or an illness affecting food intake should receive professional support rather than trying to manage recovery alone.
Does Being Underweight Affect Egg Quality or Egg Count?
The strongest evidence connects low weight and low energy availability with hormone changes and ovulation problems. Evidence that low BMI directly damages egg quality or reduces egg count is less clear.
BMI cannot measure egg quality or ovarian reserve. Age remains a major predictor of reproductive potential. Tests such as anti-Müllerian hormone and antral follicle count can help predict response to ovarian stimulation, but they should not be used as general fertility tests or as proof that natural pregnancy is or is not possible.[6]
It is therefore inaccurate to assume that every underweight woman has poor-quality eggs.
Does Being Underweight Affect Male Fertility?
Possibly, but the evidence is less certain than it is for female ovulation.
A systematic review and meta-analysis associated low male BMI with lower total sperm count and semen volume. The researchers also noted important limitations in the available studies.[7] Other research has produced mixed results.
BMI cannot diagnose male infertility. A semen analysis is the appropriate test for sperm concentration, movement and shape. Men should consider an assessment if pregnancy is delayed, particularly when low weight is accompanied by major weight loss, low sexual desire, erectile problems, chronic illness or signs of poor nutrition.
How Is Underweight-Related Infertility Assessed?
There is no single test for “underweight infertility.” A clinician may review:
- Current and previous weight
- Recent weight changes
- Food intake and exercise
- Menstrual cycles and signs of ovulation
- Stress and eating-disorder symptoms
- Medicines and medical conditions
- Pregnancy history
Testing depends on the symptoms. It may include a pregnancy test, thyroid tests, prolactin, follicle-stimulating hormone, luteinising hormone, oestradiol and a pelvic ultrasound. The 2024 ASRM guidance recommends ruling out pregnancy first when periods stop and then investigating other possible causes.[8]
If pregnancy is delayed, assessment may also include the fallopian tubes, uterus and a semen analysis. Both partners should be evaluated when appropriate. Low weight should not automatically be treated as the only explanation.
When Should You See a Doctor?
Seek medical advice if:
- Your periods have stopped for three months
- Your cycles are consistently longer than 35 days or vary greatly
- The interval between periods persistently exceeds 45 days
- Your weight loss was sudden or unplanned
- You feel faint, weak or unwell
- You have restrictive eating, compulsive exercise or symptoms of an eating disorder
- You have repeated stress fractures or signs of poor nutrition
- You have a thyroid, digestive, hormonal or chronic medical condition
For people without a known fertility problem, ASRM recommends an infertility evaluation after 12 months of regular unprotected sex when the female partner is under 35, and after six months when she is 35 or older. More immediate assessment may be appropriate after age 40.[4]
Do not wait for these time limits if periods are absent or very irregular, or if a known medical condition may affect fertility.
Frequently Asked Questions
Can being underweight cause infertility?
Being underweight can contribute to infertility when low energy availability disrupts reproductive hormones and ovulation. It does not cause infertility in everyone.
Can you get pregnant with a BMI of 17?
Yes, pregnancy is possible with a BMI of 17. However, BMI alone cannot show whether you are ovulating or receiving enough nutrition. A preconception medical and dietetic assessment is advisable, especially if your periods are irregular or your weight has fallen recently.
Can you be underweight and still have regular periods?
Yes. Some naturally slim people have regular cycles and normal ovulation. Regular periods are reassuring, but they do not rule out every possible fertility problem.
How long after gaining weight does fertility return?
There is no fixed timeline. Ovulation may return within a few months, but recovery can take longer. It depends on how long periods were absent, energy intake, exercise, stress, weight restoration and whether another medical condition is present.
Does being underweight affect IVF success?
Research is mixed. Some studies report similar pregnancy and live-birth rates in underweight and normal-weight patients, while others report poorer outcomes in certain groups. Age, nutrition, the cause of infertility and overall health are more useful than BMI alone when planning treatment.
The Bottom Line
Being underweight can affect fertility, mainly when the body does not have enough energy to maintain normal reproductive hormone signals and ovulation. The risk is greater with recent weight loss, under-eating, excessive exercise, an eating disorder or illness.
A BMI below 18.5 is a screening marker, not proof of infertility. Many underweight people still ovulate and become pregnant. If low energy availability is the cause, improving nutrition, reducing excessive exercise and treating the underlying problem may help periods and ovulation return.
Seek medical advice if your periods become irregular or stop, your weight falls without explanation or pregnancy does not occur within the recommended time.
We rely on peer-reviewed studies and reputable medical journals.