Women are often encouraged to "eat for two" during pregnancy. While this is right in terms of eating to meet both mum's and baby's needs, it doesn't mean doubling her total food and energy intake.
The amount of weight you should gain during pregnancy depends on your pre-pregnancy body mass index (BMI). If you're a healthy weight, with a BMI of 18.5 to 24.9, you should gain 11.3 to 15.9 kilograms. If you're underweight, with a BMI of less than 18.5, you'll need to gain a couple more: 12.7 to 18.1kg.
If you're overweight, with a BMI of 25 to 29.9, you should gain a little less: 6.8 to 11.3kg. For obese women with a BMI of 30 or above, the recommended weight gain is between five and nine kilograms.
These amounts increase for twin and triplet pregnancies. A woman of a healthy weight carrying twins, for example, should aim to gain around 16.8 to 24.5kg.
If this weight gain sounds excessive, keep in mind that the baby at term makes up only about one-third of this weight. The rest may be made up of other tissues and fluids, including an extra 2kg of blood, 1.5kg of breast tissue, 0.5kg placenta and 1kg amniotic fluid. All of this weight is quickly lost after birth.
It's only the 2.5 to 3.5kg of extra fat mass gained that women need to lose. This can happen quite quickly if the she chooses to breastfeed. Milk production, and hence lactation and breastfeeding, uses a lot of energy.
If a woman gains closer to 20 to 25kg during her pregnancy, however, more of that will be fat and it will be much harder to lose. It could also put her at higher risk of developing gestational diabetes and other complications during the pregnancy.
Not gaining any weight during pregnancy, or not enough, can also be dangerous.
Energy requirements vary over the pregnancy. A woman's energy requirements increase only slightly in the first trimester, so no additional energy (in terms of calories or kilojoules) is required. And only about 500g to 2kg of weight should be gained during this time.
But nutrient requirements are higher, so women need to be mindful of the quality of their diet.
In the second trimester, the energy requirements increase by about 1,400 kilojoules per day (or 300 calories). That's the equivalent of a small tub of yoghurt and a banana.
For the final trimester, the energy requirements increase again, by a total of 1,900kJ a day above pre-pregnancy requirements. So add another piece of fruit to the yoghurt and banana and you're meeting your additional energy needs.
While all nutrients are important during pregnancy, three stand out for those of us living in the western world: folate, iodine and iron.
Adequate folate levels are critical for the healthy formation of the neural tube in the developing foetus. Folate is found in green leafy vegetables and is added to commercial breads. On top of these dietary sources, antenatal guidelines recommend supplementation with 500 micrograms per day of folic acid, from 12 weeks before conception and throughout the first 12 weeks of pregnancy.
Iodine is important for hormone development and is found in dairy products, seafood, seaweed, eggs and bread. But diet alone may not deliver enough of this nutrient during pregnancy. Antenatal guidelines therefore recommend healthy pregnant women take an iodine supplement of 150 micrograms each day.
Iron helps the body manufacture red blood cells that carry oxygen around the body and can be found in red meat, fish, chicken, legumes and other plants. Unless pregnant women have a severe clinical iron deficiency, routine supplementation is not recommended. Instead, make sure you're consuming enough iron-rich foods along with foods high in vitamin C to aid absorption.
Women with clinical deficiencies or health conditions should consult their doctor prior to taking and stopping supplements.
The bottom line is that pregnant women do need to eat enough nutrients to meet both their needs and the needs of their growing baby, but the increases in energy requirements during pregnancy do not equate to "eating for two" in terms of volume.
Women need to have a sensible increase in their total energy intake to avoid excessive weight gain during pregnancy and have optimal outcomes for them and their babies.
Regina Belski is Senior Lecturer in Dietetics and Human Nutrition at La Trobe University.
Regina Belski does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.