Pregnancy and Weight Control

Women who are very overweight (or very underweight) at the beginning of pregnancy are at greater risk of developing complications during pregnancy, or in delivery. For overweight women, these include high blood pressure, hypertension or blood poisoning (toxaemia), caesarean birth, kidney ailments, gestational diabetes and longer labour.

If you’re pregnant or considering pregnancy, make sure you have a good understanding of the role of your weight in promoting a healthy pregnancy and delivery.

Weight gain during pregnancy

A mother’s pre-pregnancy weight and her weight gain during pregnancy have a major effect on the size of the baby. If the baby is too large, this may increase the baby’s risk of becoming obese later in life. It’s equally, if not more, important to reduce the risk of a low birthweight infant.

Too low a birthweight (less than 2.5 kg/5.5 lbs) increases the baby’s risk of birth defects and other health problems. The optimal birthweight is now considered to be between 2,500 – 4,000 grams.

Generally, the less weight gained during pregnancy, the lower the birthweight of the baby. This is true for all women – even obese women. Underweight women need to gain the most weight during pregnancy, and obese women need to gain the least. Some guidelines follow.

  • Ideal Weight Gain: A total weight gain of 11 – 16 kg during pregnancy is desirable. Normally, only 1.5 – 2 kg is gained in the first trimester (12 weeks). Thereafter, an average of 0.4 kg is gained each week.
  • Excessive Weight Gain: Weight gain of more than 16 kg during pregnancy may result in the baby being too large. A sudden increase in weight might result from toxaemia or excess calorie intake. Dietary changes to control weight gain are best supervised by an obstetrician and dietitian. Pregnancy is not the time for extreme dieting, as foetal growth can be impaired.
  • Inadequate Weight Gain: Weight gain of less than 7 kg during pregnancy is more likely to result in a low birthweight baby. The past practise of over-restricting food intake to limit weight gains is no longer considered healthy. Too little weight gain, even in obese women, indicates the mother’s dietary intake is too low to provide adequate nutrition for optimal development of the growing baby and the maintenance of mother’s health.
  • Teenagers are a high-risk group for low birthweight infants. Unlike older mothers, growing teenagers may need to ‘eat for two’, to accommodate their own needs, as well as those of the foetus. A weight gain of 13 – 16 kg will minimise the risk of a teenager having a low birthweight baby. Under no circumstances should a growing teenager view pregnancy as an opportunity to lose weight.
Acceptable Weight Gain During Pregnancy
Underweight: Gain 13 – 18 kg
Normal Weight: Gain 11 – 16 kg
Moderately Obese: Gain 9 – 11 kg
Very Obese: Gain 7 – 9 kg

Components of weight gain

Only 3.5 kg of the total weight gain during pregnancy is due to the developing baby itself. The remainder is comprised of vital tissues and fluids which nourish and protect the foetus before and after birth.

The components of weight gain are:

Developing baby (foetus)


Amniotic fluid

Extra breast tissue

Extra blood



Fat stores

3 – 4 kg

0.5 – 1 kg

1 – 1.5 kg

0.5 – 1 kg

1.5 – 2 kg

1 – 1.5 kg

1 – 1.5 kg

1.5 – 3 kg

Approximate total weight gain 10 – 15.5 kg

Importance of fat reserves

The fat stores the expectant mother lays down during pregnancy play an important role. They act as a safeguard against any periods of food shortage.

The energy stored in this fat is also used to produce breast-milk. Additionally, body fat insulates and protects the growing baby. Fat stores of 1.5 – 3 kg represents approximately 25,000 calories, and can subsidise the additional metabolic requirements during late pregnancy and breastfeeding. These fat stores may not be fully used, and this may contribute to residual weight gain after each pregnancy.

Calorie requirements during pregnancy

Energy requirements during pregnancy vary considerably from person to person. This reflects wide differences in your body’s energy needs, and the amount of exercise you’re performing. Extra calorie requirements are not huge, usually only 100 – 300 extra calories each day is adequate.

In women who substantially reduce their exercise, the need for extra calories may be minimal. Even towards the end of the pregnancy, only 2400 calories daily is needed by the average woman (assuming 2100 calories daily as the needs of the average non-pregnant woman).

While few extra calories are required during pregnancy, there is a greater demand for extra nutrients. It’s important to choose nutrient-dense foods, and avoid high-fat, and calorie-dense foods such as biscuits, confectionery and soft drinks.

Vitamin/mineral supplementation, particularly of iron, folic acid and zinc is usually prescribed as a safeguard.

Extra calories required

(Normal weight and activity level)

1st Trimester 100 – 150 calories

(e.g. 1 large banana)

2nd Trimester 200 – 300 calories

(e.g. 2 crackers/cheese)

3rd Trimester 300 calories

(e.g. 1 chicken sandwich)

Hazards of dieting during pregnancy

Pregnancy is not a good time to attempt weight loss, as it puts the health of your growing foetus at risk.

  • Inadequate energy intake could ‘starve’ the foetus of nutrients, especially in the third trimester, and increase the risk of a low birthweight baby.
  • Excessive weight loss can produce ketones, a by-product of fat breakdown, which may be detrimental to the baby’s development.
  • The mother runs the risk of ill-health both during pregnancy and in subsequent years. For example, inadequate calcium will increase the risk of osteoporosis (brittle bones) in later years.
  • Imbalanced nutrition during pregnancy can severely affect milk production during lactation. Adequate protein and energy is required for optimal growth of the mammary glands. Even adequate nutrition during lactation may not overcome the lack of mammary gland growth that occurred during pregnancy.

Healthy eating habits acquired during pregnancy and breast feeding can form the basis of any necessary weight loss program after the birth of the baby. If a mother breast feeds her baby for at least 4 – 6 months, she will find that by 12 months post-birth, her weight should return to pre-pregnancy levels.

Extra notes and tips

Moderate exercise during pregnancy is beneficial. Swimming, walking and jumping on an exercise bike are excellent activities which are unlikely to cause excessive stress to your body during pregnancy. Highly strenuous activity isn’t advised, as it may divert oxygen from the foetus to the exercising muscles. Lifting and carrying heavy objects should also be avoided. If any health problems are present, don’t exercise without medical advice.

During pregnancy extra iron is required to support the increase in the mother’s blood volume, and the normal growth of the foetus and placenta. Since it can be difficult to obtain the recommended amounts of iron from food alone, a daily supplement is normally prescribed during the second and third trimesters. The extra iron promotes adequate stores in the mother, to be used during breastfeeding.

Folate is required for the production of new cells in the foetus and placenta. A folate supplement is usually given in conjunction with iron.

Excess Alcohol can deplete your body of essential nutrients and cause defects in the foetus. It’s strongly advised to avoid drinking alcohol during pregnancy.

Cigarette Smoking can constrict blood vessels supplying the placenta and reduce the nutrient supply to the growing baby. Smoking will also reduce oxygen supply to the developing baby. Please don’t smoke during pregnancy.

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