Weight gain in pregnancy

Optimal birth weight

Optimal birth weight is defined as between 2.5 and 4.0kg. Achieving optimal birth weight can help to prevent childhood obesity. There are risks associated with both low birth weight and “large for gestational age” birth weight.

Low birth weight (less than 2.5kg) – Low birth weight is associated with increased risk of obesity and non-communicable disease in later life (e.g., diabetes, heart disease). One reason for this is thought to be because internal organs (e.g., pancreas) do not grow as much whilst in the womb.

Large for gestational age birth weight (over 4.0kg) – Babies who are large for gestational age have an increased risk of obesity in childhood and later life.

Factors that affect birth weight

Many factors can contribute to birth weight, including:

• Genetics
• Maternal pre-pregnancy Body Mass Index (BMI)
• Gestational weight gain (how much weight the mother gains during pregnancy)
• Diabetes during pregnancy
• Gestational age at birth

Of these, gestational weight gain is the most “modifiable” factor during pregnancy (via nutritional intake and physical activity). It is therefore important practitioners support pregnant women to achieve optimal gestational weight gain, which in turn will increase the likelihood of an optimal birth weight.

Optimal gestational weight gain

Supporting optimal gestational weight gain is important for all pregnant women.

Insufficient gestational weight gain can lead to low birth weight and/or increase the risk of anaemia in the mother.

Excess gestational weight gain can lead to large for gestational age babies, but can also lead to low birth weight if the mother is undernourished (deficient in protein or micronutrients, but not energy), or if there is placental insufficiency (where the placenta fails to transfer energy and nutrients to the fetus efficiently, leading to poor growth). For the pregnant woman herself, avoiding excess gestational weight reduces the risk of postpartum weight retention and long-term weight related health problems for the mother.

There are no specific guidelines for how much weight women should gain during pregnancy in the UK, so health professionals often refer to the USA Institute of Medicine (2009) guidelines (see table 1). These guidelines recommend optimal gestational weight gain based on pre-pregnancy BMI. It is important to note, however, these guidelines are based mostly on data collected prior to the increase of obesity in pregnancy in recent years.

Table 1: USA Institute of Medicine (2009) guidelines for optimal gestational weight gain (GWG) based on pre-pregnancy Body Mass Index (BMI). Women who achieve GWG within the specified range are more likely to have a baby with optimal birth weight.

Achieving optimal gestational weight gain

There is no need for pregnant women to ‘eat for two’ in terms of calorie intake. The department of health recommends that most women need an increase of 200 kcals per day in the third trimester only. This is the equivalent of including an extra yogurt or two slices of bread per day. Women with a low BMI (< 18.5kg/m²) will require more and women with a high BMI (> 25 kg/m²) will require less.

To encourage optimal gestational weight gain, pregnant women can be supported to:

• Focus on healthy eating that includes regular intake of reliable sources of key micronutrients (see nutrition during pregnancy section)
• Minimise ‘empty calories’ (e.g., energy-dense, nutrient-poor options that are not needed in a healthy balanced diet - such as sweets, chocolate, cakes, biscuits, sugar sweetened beverages, crisps and savoury snacks)
• Maintain an appropriate level of physical activity (see physical activity in pregnancy section)

Recommendations for pregnant women living with obesity

The National Institute for Health and Care Excellence (2010) recommends women achieve a healthy weight before conception. For women living with obesity (BMI >30 kg/m²), however, this may not be a realistic option.

It is important to take women’s pre-pregnancy BMI into consideration when providing nutritional guidance. Whilst the USA Institute of Medicine guidelines (table 1) suggest it is optimal for women who are living with obesity to gain some weight during pregnancy, more recent research (see Abayomi & Charnley (2012) for discussion) suggests it may be beneficial for those with a very high BMI to work towards weight maintenance (or even loss).

It is however important that any limitations to calorie intake are achieved in a safe way that ensures women continue to get their essential nutritional requirements.

National Institute for Health and Care Excellence (2010) guidelines recommend women should avoid ‘dieting’ during pregnancy ‘as it may harm the health of the unborn child’. This advice is largely based on evidence from the ‘Dutch famine studies’, which found that daily energy intakes in pregnancy of 600–1500 kcal were associated with premature labour, stillbirth and neural tube defects in the first half of pregnancy. Restricted energy intake during the second half of pregnancy was associated with low birth weight. It is important to remember, however, that these studies refer to extreme situations (famine) where women were severely deficient in many nutrients, not just in the amount of calories they consumed.

More recently it has been suggested that if energy intake is reduced whilst preserving all other essential nutritional requirements, this can be a safe way of restricting gestational weight gain (see Abayomi & Charnley, 2012). Furthermore, for women with a pre-pregnancy BMI >35 kg/m², avoiding gestational weight gain may lower the risk of adverse pregnancy outcomes (Narayanan et al., 2016). Other studies have found positive outcomes as a result of weight loss in pregnant women with a BMI >40 kg/m² (decreased risk of caesarean section by 24% and large for gestational age births by 11.2%) (Blomberg, 2011).

Tips for limiting gestational weight gain

Pregnant women with a high pre-pregnancy BMI can be encouraged to limit their calorie intake whilst ensuring they are getting reliable sources of essential nutrients for healthy fetal development (see ‘essential micro nutrients for fetal development’ section). This can be achieved by:

• minimising “empty calories”, such as sweets, chocolate, crisps, biscuits and sweetened drinks
• choosing low fat or low calorie options; skimmed milk, low fat yogurt/cheese, lean meat, low calorie drinks for example
• using lower fat cooking methods such as grilling instead of frying
• taking care with portion size (especially if eating out)
• ensuring a healthy diet that contains foods from all four food groups (see Eat Well Guide).

Being physically active during pregnancy may also help women with high pre-pregnancy BMIs prevent excess gestational weight gain. Physical activity should be at an appropriate level according to how active women were pre-pregnancy (see physical activity in pregnancy section). Any women living with obesity-related co-morbidities (e.g., type II diabetes) should seek specific advice from a relevant health professional (e.g., obstetrician, specialist midwife/nurse).

Recommendations for pregnant women living with underweight

Both living with underweight (BMI < 18.5 kg/m²) and insufficient gestational weight gain are associated with low birth weight in babies. It is therefore important women living with underweight (who are also less likely to gain sufficient gestational weight) are encouraged to increase their energy and nutritional intakes to achieve optimal weight gain during pregnancy.

Research shows that with every kilogram of gestational weight gain, birth weight has been found to increase by 20g.

Tips for promoting sufficient weight gain

For women who have a low pre-pregnancy BMI, optimal gestational weight gain can be encouraged by small frequent meals consisting of all the main food groups from the Eat Well guide. They should be encouraged to opt for full fat versions of foods (e.g., when choosing milk and dairy products) and avoid low calorie or lower fat versions of foods.

Whilst some physical activity may be beneficial (see physical activity in pregnancy section), this may pose risks for women living with malnutrition or extreme underweight. Any physical activity needs to be carefully managed to ensure it is not offsetting the energy taken in through calories. Therefore all women with low pre-pregnancy BMIs are advised to speak with a relevant health professional (e.g., obstetrician, specialist midwife/nurse) for physical activity advice tailored to their situation.

Key messages:

• Optimal birth weight (2.5-4.0 kg) can prevent obesity and disease later in life
• Gestational weight gain is a modifiable factor that can influence birth weight
• Optimal gestational weight gain will depend on women’s pre-pregnancy BMI
• For women living with obesity, limiting gestational weight gain may be beneficial, as long as nutritional requirements continue to be met
• Achieving sufficient gestational weight gain is particularly important for women living with underweight