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Pregnancy Bliss | Reproductive Health Hub

Weight risks in pregnancy

Is there any pre-pregnancy weight that is regarded to be a risk factor?
Both very low and overweight states are regarded as risk factors in pregnancy. There are, however, no standard cut-off points.

Many maternity units use 45 kg as the lowest border of normal, below which the woman will be regarded as underweight. The upper border of normal is 80 kg for some units and 85 kg for others. Weights above these cut-off point put the woman in the overweight category.

Some units use a calculated Body Mass Index (BMI). This is scientifically regarded as more objective though not perfect.

A BMI of less than 20 is too low and over 30 represents obesity. It is calculated by dividing the body-weight (in kg) by the square of the height in metres (kg/m2).
For instance, if the woman weighed 70 kg and her height was 1.72 m (5 ft 8 inches), her BMI will be 70/1.72 which works out at 23.64, which is within the ideal range. If, on the other hand, her height was 1.47 m (4 ft 10 inches), with the same weight, her BMI will be 32.4 which is classified as obese. You can find an instant BMI calculator here:
Underweight in pregnancy
How does underweight come about?
In the Western world, this problem is mostly a result of eating disorders such as anorexia nervosa and bulimia. These conditions mostly affect young women from the affluent social classes, and in themselves make it difficult for these young women to conceive, but some do.

Another group of underweight women are those who are chronically abusing drugs. These are likely to be underweight simply as a result of under-nourishment. Food becomes a minor priority in their lives.
In some poorer countries in the developing world, underweight may be a direct result of poverty and conse­quently poor nutrition.

Why is underweight a risk factor in pregnancy?                                
Underweight women are likely to cope poorly with pregnancy. The primary causes of their underweight are likely to become more complicated and could worsen. As far as the baby is concerned, there is increased possibility of restricted fetal growth and birth of an underweight baby, whatever the gestation at delivery. These underweight babies are at risk of several early life complications (such as hypothermia, low blood-sugar, feeding difficulties and viral infections).

The risk of losing the baby in the perinatal period is also significantly increased.

Is maternal underweight a risk factor for miscarriage?
There is no evidence to this effect.

What about maternal underweight and premature delivery?
Again no evidence exists that underweight on the part of the mother may in itself lead to premature delivery.

Can such women receive any help?
Ideally, a woman suffering from eating disorders - such as anorexia nervosa or bulimia - should be treated before they become pregnant, to allow for a normal or near normal weight at the time of conception. However, if the problem comes to light when she is already pregnant, treatment of the underlying problem goes hand-in-hand with general antenatal care. The management is multi-disciplinary for eating disorders - including obstetricians, a physician and a psychiatrist.

Those with a drug habit will also require a multi­disciplinary management, which will include, among others, the midwifery team, obstetrician and a social worker.
Central in all management regimes will be a plan to achieve appropriate nutrition.

Are there any special measures required in labour if the expectant mother is underweight?
Not really. If the fetal growth has been unsatisfactory during the pregnancy, then continuous electronic monitoring of the fetus will be necessary. Of course, intrauterine growth restriction increases the possibility of intervention in labour and delivery by caesarean section. Apart from this, the labour will be treated as any other.

Will there be any special measures after delivery?
The management of the underlying causes of the problem will most likely need to continue beyond delivery.

Underweight Overweight Obesity and delivery Weight gain BMI Calculator