If your body mass index (BMI) is within the normal range, you should put on around 13kg in weight while you are pregnant. Very little of this - only up to 1 kg - is gained during the first trimester.

However, your weight increases significantly over the next two trimesters, when you will gain about 500-700g a week, up until the final two weeks when weight gain usually tails off. Just over one-third of this additional weight is the baby, the placenta and the amniotic fluid, with the rest accounted for by the elements needed to support your pregnancy, such as your enlarged breasts and uterus, increased blood volume and body fat.

You might think that once you are pregnant, you can - and even should - 'eat for two'. However, this is a myth and it is only in the third trimester that your calorie requirements increase slightly.

In the first two trimesters, your calorie intake should remain that of a normal, healthy woman (ie 2,000 calories per day). In the last trimester, you need to increase it a little, but only by 200-300 calories. Before you rejoice and reach for the nearest bar of chocolate (a small bar is equivalent to around 250 calories), it goes without saying that this additional calorie intake should comprise nutritious foods, that is, useful calories for you and the baby!

Equally, it is hardly a green light to eat as much as you want, because 200-300 calories represents only the equivalent of a banana and a low-fat yoghurt, or of a piece of buttered toast.

There are, however, increasing numbers of women whose extremes of weight can lead to problems during or after pregnancy. If you are overweight or clinically obese (with a BMI of 26 or above), this can increases the risk of miscarriage and premature birth and of developing complications such as hypertension, gestational diabetes and pre-eclampsia.

If you are very slim, with a BMI that is below 19, and if your diet has been poor, you may be depleted in certain essential nutrients, most commonly vitamin D and iron, and find that you become exhausted by meeting the needs of pregnancy.

If you are underweight, overweight or obese, you can speak to your GP or midwife about the best way to manage your weight during your pregnancy.

Calculating your Body Mass Index (BMI)

To know if your weight when not pregnant is within the normal range, calculate your body mass index (BMI) - you can do this up until about eight weeks of gestation, as you will not yet have put on a significant amount of weight: divide your weight in kilograms by your height in metres, then divide it again.

For example, 60kg divided by 1.65m (36.3) divided again by 1.65m gives a BMI of 22.A BMI of 20-25 is considered normal. Under 20 is considered underweight and between 26 and 30 is overweight. If your BMI is over 30, you are considered to be clinically obese and you should be seen by an experienced obstetric team. Your antenatal care and birth options may be altered to ensure the best outcome.

This is an edited extract from One Born Every Minute: Expecting a Baby? by Dr Penelope Law (Quadrille, £25).

Text © 2013 Dr Penelope Law