Skinny WomanIn the UK alone there are potentially 180,000 women of child bearing age that could be having difficulty conceiving due to their low body mass. Yet the relationship between being underweight and fertility problems is not well publicised.

Society’s idea of what is normal is distorted by the media constantly bombarding us with images of super-skinny models, while being overweight enjoys all the eye-catching health news headlines. Because of this, many people do not realise the strain on their health that being underweight can cause.


What Is a Healthy Weight?

This is calculated by measuring the Body Mass Index, or BMI. A healthy weight is a BMI between 18.5 and 24.9. A BMI less than 18.5 is classified as underweight. If you are unsure how to calculate your BMI, you can use the Healthy Weight Calculator.


The Effect On Fertility of Being Underweight

Overall poor health, not enough resources and not getting the right vitamins and minerals are some of the more general ways that being underweight can affect your fertility. However, when in starvation mode (your body’s long-term response to not getting enough food) there are also some specific ways that not weighing enough can harm your chances of getting pregnant.

Underweight Effect On Menstrual Cycle

Irregular menstrual cycles are one of the most obvious effects that not weighing enough has on fertility. In extreme underweight cases, the menstrual cycle can stop completely. This is primarily caused by low levels of the fertility hormone oestrogen.

Oestrogen is produced in fat cells, so if there isn’t enough body fat, you won’t produce enough oestrogen for a normal cycle.

Underweight Effect On Ovulation

Low levels of oestrogen can also stop the ovaries from ovulating. If an egg is not released, there is nothing for the sperm to fertilise.

Underweight Effect On Endometrium

If your cycle is irregular and you do not have enough bodily resources, you are also more likely to have a thinner endometrium (this is the lining of the uterus). This needs to be at a certain thickness in order for a fertilised egg to implant and develop properly. If it is not, it can prevent you getting pregnant or increase your risk of early miscarriage.


Underweight and Pregnancy Problems

Being underweight doesn’t just affect your fertility, but can also affect any ongoing pregnancy. As well as possibly having a slightly higher risk of miscarriage, it is known that underweight women are more likely to give birth to small babies and are at increased risk of preterm birth (giving birth early before the nine months’ gestational period).


What Should I Do If I Am Underweight?

Not every skinny woman will have problems conceiving. The main way to gauge whether or not your weight may be contributing to fertility problems is by looking at your menstrual cycle. If your cycle is regular and within the normal length (21 – 35 days), then it is unlikely that your weight will be affecting your chances of getting pregnant.

However, if your cycle is irregular, your next steps will depend on why you are underweight.

Underweight Due to Medical Condition

An overactive thyroid, stress and anorexia are just some of the medical causes of being underweight. If you believe there may be an underlying medical cause to your low body weight, then make an appointment with your doctor who will be able to investigate and give you advice tailored to your specific situation.

Underweight Due to Intentionally Dieting

If you are underweight because you don’t eat very much, start increasing your meal portion sizes. However, if you are considering gaining weight it is important that this is done gradually and healthily. It should also not be seen as an excuse to treat yourself to a diet of doughnuts, cream cakes and chocolate. It remains important to eat food that is high in nutrients which will also help your chances of conception.

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