The risks of being overweight and pregnant

Being overweight or obese in pregnancy is now very common but is associated with risks and needs special consideration for antenatal care.

Around half of women of childbearing age are overweight or obese. In the UK, 21.3% of women are obese when they become pregnant and less than half of pregnant women are in the healthy weight range. Being overweight can affect a woman’s ability to become pregnant and carry her baby to term and can increase the risk of some pregnancy complications.

How can a woman tell if she is overweight?

Calculating body mass index is the most common way of finding out whether a person’s weight is within the healthy range or whether they’re underweight or overweight. Body mass index (BMI) is a measure of weight and height, so there is a mathematical calculation: divide weight in kilograms (kg) by height in metres (m) and then divide the answer by height again to get BMI.

The World Health Organization has provided categories to describe different levels of weight for all people:

BMI	Classification
less than 18.5:	Underweight
18.5 – 24.9	Normal weight
25 – 29.9	Overweight
30 – 34.9	Class I obese
35 – 39.9	Class II obese
40 upwards	Class III obese

 

Usually, the BMI should take into account the pre-pregnancy weight, or the weight at the booking appointment around 8-10 weeks. BMI calculated later in pregnancy is not reliable as the weight of the baby, placenta, womb and amniotic fluid will all make women weigh “more” but aren’t actually part of the woman’s body composition.

Losing weight before pregnancy

Being in the best possible health before pregnancy gives the best chance of becoming pregnant and having a successful pregnancy and a healthy infant. So preparing for pregnancy by losing excess weight is recommended for overweight women.

Should overweight women try to lose weight once they find out they’re pregnant?

Currently there is no evidence to show that weight loss during pregnancy is beneficial or safe. Because of this, women are not advised to try and lose weight while they are pregnant. There is some evidence that during pregnancy women who are obese may not gain as much weight as normal-weight women, and some obese women may be more likely to lose weight. It is not clear why this happens, but it may be that adopting a healthier lifestyle during pregnancy influences weight change in obese women differently than for women who are at a healthy weight.

Anti-obesity and weight loss drugs should be avoided in pregnancy. For some drugs the safety has not been tested in pregnancy, and for others there are links to cleft lips or low birth weight.

How much weight gain is normal during pregnancy?

Although losing weight during pregnancy is not recommended, excessive weight gain can be harmful. The UK National Institute for Health and Care Excellence do not provide estimates of what constitutes ‘usual weight gain’ because the evidence around weight-gain ranges during pregnancy is limited. In the US, the Institute of Medicine (IOM) has suggested healthy amounts of weight gain based on pre-pregnancy weight:

These recommendations were originally developed for underweight women to ensure adequate nutrition to prevent the serious risk of fetal growth restriction for which there is strong evidence. Over time they came to include recommendations for overweight and obese women, though the evidence here is not as strong and is based mainly on reducing macrosomia (large babies), caesarean section rate and post-pregnancy weight retention. It does not take into account reducing other serious risks of obesity such as gestational diabetes or pre-eclampsia.

It is clear, however, that the amount of weight a woman gains in pregnancy can vary but should be gradual and correspond with the small weight increases of the growing baby and placenta and with minimal laying down of maternal fat. There is no need to increase calorie consumption so no need to ‘eat for two’ or to drink full-fat milk because energy needs change very little in the first six months of pregnancy and increase only slightly in the last three months (and then only by around 200 calories per day).

Overweight while pregnant: what are the risks?

Most overweight and obese women will not have any problems during pregnancy. However, compared to women who are a normal weight, obese women are at higher risk of:

• Miscarriage or stillbirth
• Several pregnancy-related medical conditions including gestational diabetes and high blood pressure and pre-eclampsia
• Venous thromboembolism (VTE) (blood clots)
• Labour and delivery interventions including induction of labour, prolonged or dysfunctional labour and caesarean section
• Anaesthetic risks, including higher risk of needing the epidural to be re-sited
• Depression and anxiety in pregnancy
• Slower initiation of milk production and breast-feeding

Importantly, the 2015 MBRRACE-UK (Mothers and Babies: Reducing Risk through Audits and Confidential Enquiries across the UK) review into maternal deaths reported that 30% of women who died during or shortly after pregnancy were obese and 22% were overweight. The numbers of women and the absolute risk is very very small, but still should not be ignored.

Pregnancy is sometimes thought of as a ‘test’ for future health. Obesity outside of pregnancy increases the risk of several medical conditions including diabetes (high blood sugar levels) and heart disease. All women who develop gestational diabetes or high blood pressure irrespective of their weight are more likely to develop these medical conditions in later life.

Does being overweight during pregnancy pose any risks to the baby?

The infants of obese pregnant women are at higher risk of:

• Prematurity (delivery before 37 weeks)
• Congenital anomalies including spina bifida and heart defects
• Macrosomia (being overweight)
• Neonatal death
• Obesity in childhood

Compared to a baby born at term, a baby who is born early (before 37 weeks of pregnancy) is at a higher risk of being admitted to the neonatal unit, needing supplementary feeds, and developing various medical conditions including respiratory problems, such as asthma, and growth and developmental delays. Infants born to obese mothers are more likely to be heavier and to have greater levels of body fat compared to infants born to mothers with a BMI in the healthy range. Babies born heavier have a higher risk of obesity and diabetes in later life.

Are there extra tests or monitoring required for overweight women?

Women who are overweight may require extra care during their pregnancy, but this will depend on how overweight a woman is and if she develops a medical condition (such as gestational diabetes or preeclampsia) or complication (such as a larger than average infant). Women who are obese may be offered attendance at a specialist clinic where they can see health professionals (for example, an obstetrician, anaesthetist, midwife and dietician) who are knowledgeable of the risks linked to maternal obesity, what can be done to minimise these risks, and how best any complications should be managed.

It may be that conventional methods of checking fetal growth, such as measuring the bump, are less reliable or not possible due to extra weight around the abdomen. These women may need extra scans to check on the baby and sometimes ultrasounds can be more difficult due to more fat tissue under the skin.

Things to know:

• Overweight and obese women are recommended to take a higher dose of folic acid (5mg) and vitamin D supplements in pregnancy.
• If a woman has a BMI >35kg/m2 and another risk factor for pre-eclampsia (age over 40, family history of pre-eclampsia, first pregnancy or multiple pregnancies), then she may be recommended to take 75-150mg aspirin from 12 weeks until the birth of the baby
• All women with a BMI >30kg/m2 should be screened for gestational diabetes
• Women with a BMI >35kg/m2 should have extra scans to check the fetal growth
• If a woman has a BMI >35kg/m2, then it is usually recommended that she deliver in an obstetric unit due to the higher chance of complications in labour
• All women with obesity are recommended to have an active third stage of labour to reduce the chance of bleeding post-delivery

Weight loss following pregnancy

Women should aim to start losing weight gained during pregnancy within six months of childbirth; when exactly a woman is ready will depend on her unique situation. All women should eat a healthy balanced diet, undertake a moderate amount of physical activity (20 to 30 minutes three to four times a week) and lose weight gradually (0.5-2lb per week). Weight loss goals should be realistic and weight loss programmes can provide support and encouragement and may increase the chance of success. It is also good to remember that breastfeeding can help weight loss and will benefit baby and that moderate-intensity physical activity and gradual weight loss will not affect a mother’s ability to breastfeed or the quantity or quality of breast milk. Ideally, women should return to their pre-pregnancy weight before embarking on another pregnancy.

Main Points

• Around half of childbearing age women are overweight or obese. Being overweight can increase the risk of pregnancy complications.
• Calculating body mass index (BMI) is the most common way of finding out whether a person’s weight is within the healthy range.
• There is currently no evidence to show that weight loss during pregnancy is beneficial or safe. Therefore, women are not advised to lose weight while pregnant.
• The amount of weight a woman gains during pregnancy can vary, but it should be gradual and should correspond with the weight increases of the baby.
• Energy requirements change very little in the first six months of pregnancy and increase only slightly in the last three months (and then only by around 200 calories per day), so there is no need to increase calorie consumption.
• There are specific recommendations for pregnancy and labour for women with obesity
• Women should aim to start losing weight gained during pregnancy within six months of childbirth, although the exact time to start losing weight is individual and based on the woman’s situation.