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BMI and fertility: how does obesity affect your ability to become a mother?
When we start on the path to motherhood, we rarely think that something as everyday as our weight can become an obstacle to becoming a mother. However, being underweight, overweight or obese can alter our hormonal balance, influence ovulation and reduce the chances of pregnancy, both naturally and through assisted reproduction.
In this article, we talk to Dr Marta Zermiani, a fertility specialist at a fertility specialist in Madrid, about the relationship between BMI (Body Mass Index) and fertility. We answer all your frequently asked questions about weight and give you tools to restore your reproductive health if your weight is affecting your ability to conceive.
- 1. What is BMI and why is it so important for fertility?
- 2. BMI calculator
- 3. BMI Calculator
- 4. What is the ideal BMI for getting pregnant?
- 5. How does low weight affect fertility?
- 6. How does being overweight or obese affect female fertility?
- 7. Does obesity also affect male fertility?
- 8. What can I do if my BMI is not in the ideal range?
- 9. How much should I weigh to get pregnant?
- 10. Is it possible to get pregnant when you are overweight?
- 11. Tips for achieving pregnancy with an abnormal BMI
- 12. FAQs about BMI and infertility
- 13. How does BMI affect female fertility??
- 14. What BMI is considered ideal for getting pregnant?
- 15. Can I undergo IVF treatment if I am overweight or obese?
- 16. What is the maximum BMI allowed for IVF?
- 17. Can extreme thinness or a low BMI cause infertility?
- 18. Can obesity also affect male fertility?
- 19. Is a man’s BMI taken into account for IVF or artificial insemination?
- 20. Can weight influence sperm quality?
- 21. Do people usually gain weight after IVF or artificial insemination?
- 22. Our fertility treatments
- 23. Where can I find Vida Fertility centres in Spain?
What is BMI and why is it so important for fertility?
BMI, or body mass index, is a formula that relates weight and height to classify body status: underweight, normal weight, overweight or obese.
- BMI = weight (kg) / height² (m²)
BMI calculator
BMI Calculator
BMI Classification:
- < 18.5: Underweight
- 18.5 – 24.9: Normal weight
- 25 – 29.9: Overweight
- 30 – 34.9: Obesity class 1
- 35 – 39.9: Obesity class 2
- ≥ 40: Obesity class 3
*The above values are for guidance only; always consult a medical specialist about your case.
According to the WHO, these are the general ranges:
BMI values table
| BMI | Classification |
|---|---|
| Less than 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25 – 29.9 | Overweight |
| 30 or more | Obesity |
A BMI outside the healthy range can interfere with the hormonal, ovulatory and metabolic processes that are key to conception.
What is the ideal BMI for getting pregnant?
Fertility specialists in and assisted reproduction specialists in Alicante agree that the ideal range between BMI and fertility is between 18.5 and 24.9. This does not mean that it is impossible to conceive outside this range, but it does mean that the chances are lower.
In terms of fertility, maintaining a healthy BMI can:
- Improve ovulation.
- Improve the quality of oocytes.
- Reduce the risk of miscarriage.
- Facilitate embryo implantation.
How does low weight affect fertility?
We often talk about obesity, but low weight is also a limiting factor when trying to conceive.
A BMI below 18 or excessive weight loss can also have a negative impact on fertility in both women and men.
This low weight may be related to insufficient nutrition, health disorders that increase the body’s energy requirements, or high-intensity physical exercise.
In women who are underweight or have a BMI <18.5, the following are observed:
- Anovulation or irregular cycles.
- Amenorrhoea (absence of menstruation).
- Low oestrogen levels.
- Decrease in endometrial thickness.
- Increased risk of miscarriage.
In addition, there may be significant nutritional deficiencies (iron, zinc, vitamin D) that directly affect egg development and the uterine environment.
Effects of low weight or low BMI in men:
Low weight in men can affect semen quality, causing a lower sperm concentration and reduced motility, which hinders fertilisation.
How does being overweight or obese affect female fertility?
Being overweight and, in particular, obese, causes a hormonal imbalance that affects ovulation, the endometrium and the response to assisted reproduction treatments.
Among the most common consequences are:
- Increased androgens and insulin resistance.
- Polycystic ovary syndrome (PCOS).
- Implantation failure and low endometrial receptivity.
- Lower response to ovarian stimulation.
- More complications during pregnancy: gestational diabetes, pre-eclampsia, premature birth, among others.
In addition, overweight women have a lower pregnancy rate, as problems such as the following may arise:
- Possible resistance to gonadotropins, which means higher hormone doses are needed during ovarian stimulation.
- Risk of asynchrony in follicular development, which can result in the retrieval of immature eggs.
- Reduced ovarian response to stimulation.
- Higher probability of cycle cancellation if the expected objectives are not achieved.
- Increased risk of complications in the follicular puncture procedure.
- Lower fertilisation rates compared to patients with a better response.
- Possibility of obtaining lower quality embryos.
Does obesity also affect male fertility?
Yes. Obesity is also a cause of male infertility. Although it is often overlooked, overweight men can also have their fertility affected:
- Decreased sperm count.
- Reduced motility and morphology.
- Altered sperm DNA.
- Decreased free testosterone.
- Erectile dysfunction.
- Risk of oligozoospermia, asthenozoospermia and teratozoospermia.
That is why, at Vida Fertility, we approach fertility as a joint process for the couple.
What can I do if my BMI is not in the ideal range?
At Vida Fertility, we support you with a multidisciplinary approach. Here are some practical recommendations:
1. Calculate your current BMI
We provide you with this BMI calculator for adults, which will allow you to find out your index easily and with reliable results.
2. Do not make drastic decisions
Losing or gaining weight too quickly can be counterproductive. Consult a nutritionist specialising in fertility.
3. Personalise your treatment
We adapt each protocol (AI, IVF, egg donation) to your metabolic and hormonal situation to maximise your chances.
4. Trust a specialised team
At Vida Fertility, we work from an integrative medicine approach, collaborating with endocrinologists, psychologists and nutritionists to help you achieve your dream of becoming a mother.
How much should I weigh to get pregnant?
There is no universal ‘ideal’ weight, but rather a balance between weight and height that is measured by calculating your BMI. However, as a general guideline:
- Our BMI should be between 18.5 and 24.9.
- The most important thing is to have an ovulatory cycle, with good ovarian reserve and a receptive endometrium.
Is it possible to get pregnant when you are overweight?
Yes, of course. Many overweight women get pregnant, both naturally and with fertility treatments. But you have to assume that:
- It may take longer.
- The response to medication may be different.
- It is essential to control risk factors before pregnancy (glucose, blood pressure, thyroid).
Having a healthy BMI does not guarantee pregnancy, but it does significantly improve medical options and protocols.
Tips for achieving pregnancy with an abnormal BMI
When a couple has a BMI outside the healthy range and wants to have a child, it is best to consult a nutrition specialist who can guide them towards a healthy weight and a safer pregnancy.
In cases of obesity, it is also advisable to see an endocrinologist for stricter and more personalised monitoring.
In many situations, a moderate loss of between 5 and 10% of initial body weight, achieved through dietary changes and the incorporation of regular physical activity over a few months, may be sufficient to improve reproductive function and promote natural pregnancy.
Some practical recommendations for achieving a balanced BMI and improving fertility are:
- Adopt a healthier lifestyle.
- Follow a varied and balanced diet, rich in fruit, vegetables, fish and legumes.
- Start taking folic acid supplements before conception.
- Exercise moderately and consistently.
- Avoid tobacco, alcohol, excessive caffeine and exposure to toxins.
In summary, both being overweight and extremely thin can affect fertility, but in many cases these problems can be reversed with changes in lifestyle and diet.
If you have been trying to conceive for between 6 months and a year without success, consult us about your case. We have the best specialists in assisted reproduction and cutting-edge technology in our fertility laboratories.
At Vida Fertility, we offer you a comprehensive diagnosis of your overall health and treatment tailored to your situation, whatever your starting point.
Book your first appointment with our specialists. We will assess your case and medical history and design the best plan to help you become a mother.
FAQs about BMI and infertility
How does BMI affect female fertility??
Dr Marta Zermiani answers:
Body Mass Index (BMI) influences multiple aspects of female infertility and male infertility :
- It affects hormone regulation, especially oestrogen, progesterone, insulin, and thyroid hormones. A high BMI can increase insulin resistance, which disrupts ovulation.
- In obese women, it can increase extra oestrogen levels (due to the aromatisation of androgens in adipose tissue), which interferes with hormonal feedback and can inhibit ovulation.
- In women with very low BMI, adipose body mass may be insufficient to generate enough oestrogen, which can lead to amenorrhoea (absence of menstruation) or anovulatory cycles.
- BMI also influences uterine receptivity, endometrial quality and the follicular environment, which can affect embryo implantation and development.
What BMI is considered ideal for getting pregnant?
Dr Marta Zermiani replies:
Although there is no exact figure for everyone, the reference values for a woman seeking pregnancy are usually between 18.5 and 24.9 kg/m².
- Within this range, the risks of hormonal imbalances, anovulation, complications during treatment and pregnancy are lower.
- However, women with a slightly higher BMI (moderate overweight) can have good results if they combine medical treatment with lifestyle improvements.
Can I undergo IVF treatment if I am overweight or obese?
Dr Marta Zermiani replies:
Yes, it is possible. But there are several nuances:
- In IVF (in vitro fertilisation) treatment, being overweight or obese can reduce the chances of success, as increasing the dose of hormone medication needed increases the risk of complications such as hyperstimulation.
- This does not mean that it is impossible. Many overweight or obese women achieve pregnancy through IVF, however, it has been shown that there is a higher success rate in patients with a normal BMI.
- It is important for the medical team to evaluate each patient individually (hormonal profile, metabolic health, presence of other conditions such as diabetes, hypertension, metabolic syndrome).
What is the maximum BMI allowed for IVF?
Dr Marta Zermiani replies:
There is no universal consensus, and it depends on each clinic, but in many fertility clinics in Spain we have internal policies that consider a BMI of 35-40 kg/m² to be the limit for full IVF treatment, especially if there are risk factors (cardiovascular, metabolic).
At Vida Fertility, we evaluate each case individually; if the BMI is high but the patient is in good general health and there are no risks, we do not rule out treatment, although we always recommend and support interventions to improve lifestyle and optimise results.
Can extreme thinness or a low BMI cause infertility?
Dr Marta Zermiani replies:
Yes, a very low BMI can significantly affect fertility.
- Extreme thinness is often associated with very low fat mass, which reduces the oestrogen levels necessary for a regular menstrual cycle.
- It can induce amenorrhoea, anovulation, reduction in functional ovarian reserve, affect oocyte quality, but also implantation.
- In addition, poor nutritional status can affect essential micronutrients (iron, vitamin D, B vitamins) and cause metabolic dysfunction.
Can obesity also affect male fertility?
Dr Marta Zermiani replies:
Yes, as we mentioned at the beginning of the article, in men, excess weight can affect:
- Sperm quality (quantity, motility, morphology).
- Sperm production, by altering sex hormones (low testosterone, high oestrogen).
- Testicular temperature, as excess fat in the groin/perineal region can increase local temperature, which impairs spermatogenesis.
- Increased oxidative stress and inflammatory markers that affect sperm quality.
Is a man’s BMI taken into account for IVF or artificial insemination?
Dr Marta Zermiani replies:
Yes. Although traditionally more emphasis has been placed on women, men are now also considered:
- A very high BMI may require adjustments to sperm (processing) and may increase the risk of fertilisation failure or suboptimal embryonic development.
- In artificial insemination, if the semen analysis shows abnormalities and the man is obese, lifestyle improvements, diet and exercise are recommended; this can improve results before treatment.
Can weight influence sperm quality?
Dr Marta Zermiani replies:
Absolutely yes:
- Obesity is associated with greater sperm DNA damage and greater DNA fragmentation, which translates into implantation failure and poorer embryo quality.
- It can also affect sperm morphology and motility.
- In addition, nutritional deficiencies in micronutrients (zinc, vitamin D, antioxidants) are often present in both poor diets and obesity, which exacerbates the problem.
Do people usually gain weight after IVF or artificial insemination?
Dr Marta Zermiani replies:
This is a question we hear a lot in our clinic, and the truth is that it can happen, although not in all cases:
- Some hormonal treatments (e.g. those used in ovarian stimulation) can cause fluid retention.
- Stress, changes in habits during treatment, decreased physical activity, cravings, and side effects of medications can contribute to weight gain.
- However, with proper nutritional monitoring, exercise, and psychological support, this weight gain can be minimised or avoided.
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Where can I find Vida Fertility centres in Spain?
📍 C/ Palermo 15, Hortaleza, 28043 Madrid
📞 Phone number: +34 919 29 83 23
🕒 Opening hours: Monday to Friday from 8am to 7pm
📍 Av. Óscar Esplá 1, Bajo, 03008 Alicante
📞 Phone number: +34 919 29 83 23
🕒 Opening hours: Monday to Friday from 8am to 7pm
ℹ️ The information published on Vida Fertility is reviewed by our fertility specialists and medical team, following up-to-date scientific criteria and evidence-based medicine. Our team is a member at scientific societies such as SEF, ESHRE and SEGO.
