We all know the importance of maintaining a healthy weight for our body size and shape. After all, maintaining a healthy weight can help us to keep our heart pumping, our blood pressure regulated, and our bones and joints pain free. But did you know that weight also plays a significant role in fertility?

If you are overweight or underweight, your body may be having trouble regulating its natural cycle. Your body needs to be of the right weight in order to produce the appropriate amount of hormones to regulate ovulation and menstruation.

If you are overweight or underweight, your body can start to experience problems with these natural fertility cycles, impacting your ability to become pregnant. In fact, more than 12% of all infertility patients suffer from weight-related infertility.

Women who are underweight are at risk of compromising their fertility cycle. If you have less than 22% body fat, your body will not receive enough oestrogen and ovulation could stop.

Many women already know that if they become too thin, their periods stop, which means they are not ovulating and therefore cannot become pregnant. Women with eating disorders, such as anorexia nervosa and bulimia nervosa, are often extremely underweight, but a huge number of women in the UK are constant dieters and may keep their weight just on the limit of being healthy. Also, many of the diets that they follow are not adequate and can result in low dietary intakes of essential nutrients for a healthy reproductive cycle and therefore fertility and pregnancy.

Evidence has also recently emerged about reduced fertility in overweight women. It appears the risk of infertility increases with the degree of obesity: in other words, the greater the weight, the bigger the problem.

One Australian study showed that when overweight infertile women successfully followed a weight loss and exercise programme for six months, their periods returned and most went on to have healthy babies. This is because women who are overweight tend to have a higher percentage of fat on their bodies and as fat cells produce oestrogen, some overweight women produce levels of oestrogen that are far too high. This can negatively influence menstruation and ovulation, making it difficult to become pregnant.

Studies have also shown that being significantly overweight may also affect how well women respond to certain fertility treatments, such as IVF.

It is not only being the right body weight that is critical for fertility, the amount of body fat you have is just as important. In normal adult women, fat comprises about 28 per cent of body weight and, if it drops below 22 per cent, then ovulation will stop.

Women with an average or above average body weight or who exercise very rigorously may have a lower body fat and a higher muscle content, which may lead to their periods becoming irregular or stopping altogether. Sensible advice for these women would be to reduce their exercise until their body fat returns to the normal range. Many gyms have simple devices that can check your body fat level.

If you are concerned that your weight may be affecting your fertility, don’t be tempted to lose weight quickly. A slow and steady approach, no more than 1 or 2 lbs a week, has more lasting effects. Make sure your diet contains all the nutrients you need but with fewer calories and you can do this by limiting consumption of saturated fat, sugar and refined carbohydrates and including more fruits, vegetables and legumes into your diet.

You should also exercise to boost your metabolism so that you can eat plenty of health boosting nutrients and still lose weight. Getting all the nutrients you need is crucial for you and your baby to be, so as well as additional folic acid, which you should be taking anyway, consider taking a multi-vitamin and mineral designed for fertility (see Resources, page 41).

A healthy body weight is crucial for optimum fertility. But before you step reluctantly on the scales again, here’s something to bear in mind: Your ideal weight for fertility is probably heavier than you think and as far as your fertility is concerned you can be too thin. So if you do need to gain weight eat plenty of whole grains, vegetables, fruits, oily fish and nuts and seeds.

You may be tempted to eat sweets and fatty foods to bump up your calorie intake but try to resist. They are just ’empty calories’. These food choices will fill you up but they won’t give your body the nutrients you and your future baby need to thrive. When it comes to weight and your fertility, as in all things, it is all a question of healthy balance. You shouldn’t be too heavy or too thin but somewhere in between.

A new study has found that dieting provokes cravings for ‘forbidden’ foods like chocolate.

Psychologists at Hertfordshire University found that dieting is counterproductive as it increases cravings for foods slimmers tend to avoid. So don’t diet but lose weight by eating healthily and also exercise.

Just fifteen minutes of brisk activity a day could halve the risk of children becoming obese, say researchers. They discovered a small amount of moderate exercise could have far reaching benefits.

The number of fat children has doubled in ten years with one in four aged 11-15 now classified as obese. Almost half of girls are obese or overweight. Junk food, sedentary lifestyles and the couch potato use of TV as an ‘electronic babysitter’ are blamed for creating the problem. At the same time, levels of inactivity have soared as many children no longer take part in competitive sports at school and playing fields have been sold off.

The Bristol University Children of the 90s research project monitored 5,500 12-year-olds. It found 15 minutes of extra activity per day cut the risk of becoming obese by more than half in boys and at least 40 per cent in girls.

This study provides some of the first robust evidence on the link between physical activity and obesity in children. We know that diet is important – but what this research tells us is that we mustn’t forget about activity. It’s surprising how even small amounts of exercise appear to have dramatic results. In the latest study, published in March 2007 in the journal PLoS (Public Library of Science) Medicine, children were given a monitor to wear for up to a week which kept track of their movements. The research showed that the level of intensity of the activity was critical.

Moderate to vigorous activity was much more effective, which means exercise that makes a child get a little out of breath, but the study also indicated that it may be something quite small that makes a difference – such as how children get to school, whether they are walking, cycling or being driven in a car.