How much energy does a pregnant woman need to include in her diet?

Recommended weight gain during pregnancy and the agreed amount of energy needed in the diet will be different for each woman - both will depend on the woman's body 'size' before pregnancy. The benefits of achieving an ideal weight gain during pregnancy are for the mother and her baby.

Weight gain in pregnancy is made up of: 

• the developing foetus, placenta and amniotic fluid
• increases in volume of the woman's own body (uterus, breasts, blood and cell fluid), and 
• increases in the woman's fat stores.

A short history of advice given to healthy pregnant women

Many years ago, pregnant women were advised to eat for two. Then, in a complete change, they were advised to restrict food to avoid excess weight gain in order to prevent toxaemia (pre-eclampsia), difficult births and maternal obesity.

However, the idea of food restriction was challenged in the 1960s when the detrimental effects of a low birthweight on infant health were recognised. Pregnant women were then advised against severe dietary (calorie) restriction and routine limitations for weight gain.

In the 1970s, a study managed to establish the 'normal' weight gain during pregnancy and the weight gain associated with the best outcomes for mother and baby. The UK study (by Hytten & Leitch) of over 3800 pregnant women who ate without restriction found the total weight gain during pregnancy (gestational weight gain; GWG) of 12.5kg was associated with the best outcome in terms of birth weight, infant survival and rates of pre-eclampsia.

In 1990, a US report for the Institute of Medicine (IOM) Food and Nutrition Board (updated in 2009) returned to the relationship between pregnancy weight gain, the best outcome for infants (in terms of birth weight) and for mothers (in terms of delivery complications and retaining the extra bodyweight after giving birth).

Importantly, it showed that, because the weight gained during pregnancy actually influences the woman's energy requirement, maternal body mass index (BMI) should be taken into account when making dietary recommendations for pregnant women. Thus, recommendations then became based on BMI (= weight divided by height squared; see our BMI calculator).

These total weight gain ranges according to pre-pregnancy BMIs are known as the IOM recommendations (Updated in 2009). They are the basis for the New Zealand Ministry of Health recommendations for weight gain in pregnancy. (It should be noted that BMI tends to relate to fat mass on the body but, for active or more muscular women, BMI may overestimate their fat mass. People of Asian origin tend to carry a higher fat mass for a given BMI, and those of Polynesian origin have a proportionately lower fat mass for a given BMI. The above recommendations may be affected by these ethnic differences.)

Food and Nutrition Guidelines for Healthy Pregnant and Breastfeeding Women in New Zealand

According to the New Zealand Guidelines, on average, energy requirements increase by 12% during pregnancy. In the first trimester, there is little or no need for extra energy but by the third trimester, the woman has a higher rate of resting metabolism due to the rapidly growing baby and the effect of her own extra bodyweight, and so she will need extra calories in her diet. However, this need may be offset (to a very small extent) because a previously active woman may, unsurprisingly, become less active later in pregnancy and burns less energy through physical activity.

Fortunately, for well-nourished women, the optimum weight gain in pregnancy can be achieved with widely varying energy intakes. The right amount of energy in the diet is much more important for women who are underweight or overweight before pregnancy.

In undernourished women and pregnant adolescent women (who are still growing), if there is a shortage of calories in the diet, the mother's body will take the nutrients it needs first, compromising the growth of the baby, and raising the chances of a pre-term delivery. Too much weight gain in pregnant women who were already overweight can increase the chances of having a very large baby (over 4kg). If you have a BMI that is below 19.8 or above 30 at the start of pregnancy it is recommended you receive advice from a registered dietitian.

Pre-pregnancy weight and the weight gained during pregnancy are related to some extent. It seems young, short, thin women (and smokers) gain less weight, while tall, heavy women (especially if having a first child, or they have high blood pressure) gain more weight in pregnancy. Also, the more weight gained the more seems to be retained after the pregnancy - this is thought to because, once over the recommended weight gain, the weight that goes on is mainly fat.

For women in the ideal range of BMI before pregnancy, the New Zealand Guidelines do give recommended average increases in energy intake for the three trimesters of pregnancy (1st, no extra energy; 2nd, +1400kJ or + 340kcal per day; 3rd, +1900kJ or +450kcal per day), but care needs to be taken in applying these to individual women.

Rate of weight gain in pregnancy

• Weight gain is much lower in the first than the second and third trimesters
• Weight gain in the first half of pregnancy can affect the baby's growth throughout pregnancy
• The first half of pregnancy is considered the preparation for later growth of the baby
• 90% of the baby's growth happens in the second half of pregnancy
• Maternal fat stores (average 3.3kg) are mainly gained between weeks 10 and 30
• For women with a normal BMI before pregnancy, in the second and third trimesters a weight gain of 0.4kg per week is about right (0.5kg for underweight; 0.3kg for overweight; obese women need individualised advice).

Your lead maternity carer or dietitian should be able to advise you on the energy requirements that are right for you. They can also ensure the quality of your diet is the best it can be for your pregnancy - quality is every bit as important as quantity. (See our topic Eating well in pregnancy.)

The advice to pregnant women from dietitians is to eat a variety of foods according to their appetite and not to restrict their energy intakes unless they are already obese or gaining weight at a higher rate. Regular monitoring of weight changes should be performed as part of antenatal care accompanied by individual recommendations for energy intake with particular focus on energy intake in the second and third trimester.

This article is based on one written by Lyn Lloyd for New Zealand Doctor newspaper. Lyn is a dietitian with Auckland Dietetic Consultants Mercy Specialist Centre. IOM recommendations updated April 2010.

Food and Nutrition Guidelines for Healthy Pregnant and Breastfeeding Women in New Zealand - pdf file (645k)