Women's health: Obesity in pregnancy

Being a healthy weight is important for general wellbeing for everyone, but especially before and during pregnancy

Women who are obese when they become pregnant may face increased risks to their own health, and the health and future wellbeing of their unborn baby. Management of obesity during pregnancy also places a large burden on healthcare resources.

The increasing prevalence of obesity in the UK is a major public health concern for all. It is estimated that 24 million adults in England are overweight or obese.¹ Women of childbearing age are not exempt from this problem. Figures suggest that up to 50% of women of childbearing age are overweight, with a body mass index (BMI) over 25kg/m2 .²

The prevalence of obesity (BMI>30kg/m2) in pregnancy has increased dramatically, from 9-10% in the 1990s to 16-19% in the 2000s.^2,3 This is of great concern, since a report by the confidential enquiry into maternal deaths4 found that 28% of mothers who died were obese.

In addition, being either overweight or underweight are significant factors that influence both maternal and foetal wellbeing and growth.⁵ It has been suggested that there is a v-shaped pattern of risk with lowest risk among women with BMI of 23kg/m2 and higher risk at higher and lower BMI.

COMPLICATIONS DURING PREGNANCY

Obesity is a known risk factor for a number of pregnancy complications with long and short term effects for the mother, foetus, newborn and even the next generation of mothers and children. Recent studies 6,7 reported that women who were obese in early pregnancy had nearly double the risk (1.6%) of foetal or infant death compared with normal weight women (0.9%). Being overweight or obese prior to and during pregnancy, is linked with increased risk of almost all pregnancy complications, including gestational diabetes, hypertension and pre-eclampsia, and delivery of a large baby, as well as increased risk of miscarriage, stillbirth and infant death. Obese pregnant women are more likely to have a complex birth, and are more likely to have their baby delivered by emergency caesarean section. They also often require specialist equipment during delivery, such as large operating tables. A full list of the increased risks is shown in Box 1.

There is evidence of an association between maternal obesity and a higher risk of congenital abnormalities, including neural tube defects, spina bifida, cardiovascular anomalies, cleft lip and palate and hydrocephaly.⁹ Factors contributing to this include nutritional deficiencies, high blood glucose, and also technical difficulties during ultrasound scanning, which may lead to a lower detection rate of congenital problems. The RCOG also recommends that obese women planning a pregnancy should take a 5mg supplement of folic acid daily (general advice for women planning, and in the earliest stages of pregnancy suggest only 400mcg daily), as there is an increased risk of NTD.

Gestational diabetes (GDM), the clinical manifestation of glucose intolerance, is more common in obese mothers, and can increase risk of macrosomia. Gestational diabetes involves both decreased insulin sensitivity and insufficient insulin secretion, resulting in hyperglycaemia. Obese women, pregnant or not, are generally more insulin resistant than non-obese women. Since there are significant decreases in insulin sensitivity by the end of pregnancy consideration for screening for GDM is important. NICE Guideline 63 suggests that all pregnant women with BMI over 30 should be screened for GDM.¹⁰

Even in normal weight women, excessive weight gain during pregnancy is associated with increased prevalence of GDM, failed inductions, lacerations, caesarean deliveries and post-partum infections. Furthermore, excessive weight gain does not improve foetal growth or gestational duration, but can contribute to post-partum long-term obesity, which increases risk of developing type 2 diabetes after pregnancy as well.

In recognition of the substantial risk of obesity in pregnancy, NICE guidelines11 have been produced which indicate that health professionals should encourage obese women to reduce weight before becoming pregnant.

PREGNANCY PLANNING

A healthy weight during the periconceptual months is critical; being too underweight or too overweight can adversely affect fertility and obesity is known to be associated with subfertility in women with and without polycystic ovary syndrome.¹² Research has shown that the healthiest pregnancy is associated with a body mass index (BMI) of 23kg/m2.

Since obesity is a risk factor for a number of complications in pregnancy, obese women should be encouraged to achieve a healthy weight before pregnancy. Effective contraception can help a woman to delay pregnancy whilst losing weight, but obesity may affect the efficacy of some contraception.¹²

Awareness of the increased risk to the mother and unborn child of obesity among women of reproductive age is an important public health message to convey. However, effective weight loss programmes are only appropriate for pregravid women; weight loss during pregnancy may be potentially harmful. In particular, pregnant women should be advised against rapid weight loss or crash dieting as this could lead to ketoacidosis, which can lead to neonatal death or cognitive impairment later in the child's life.

OPTIMAL WEIGHT GAIN

High and low pre-pregnancy BMI put a pregnancy at increased risk. The components of normal weight gain during pregnancy are outlined in Box 2.

The foetus accounts for 27% of total weight gained, amniotic fluid for 6% and the placenta 5%. The remainder is accounted for by increases in maternal tissues. Estimates suggest that approximately 5% of the total weight gained occurs in the first trimester, with the rest gained at a rate of approximately 0.45kg (1lb) per week through the second and third trimester.

Dietary energy requirements do increase during pregnancy, but only by around 200kcal for the last trimester only, consequently, the message 'eat for two' needs to be quashed.

The amount of weight gained during pregnancy is variable and there is ongoing debate about the amount of weight that women should gain during pregnancy. The WHO Collaborative Study on Maternal Anthropometry and Pregnancy Outcomes14 showed that birthweight of 3.¹ - 3.6kg was associated with optimal outcomes for both mother and foetus. The range of normal maternal weight gain for this outcome was 10-14kg (mean 12kg).

There are currently no official recommendations for pregnancy weight gain in the UK, although the average weight gain is reported to be 11-16kg.¹⁵ For twin pregnancies, greater weight gain is observed, with a different pattern of weight gain - typically more is accrued in early pregnancy.¹⁶

In the absence of UK guidelines on optimal weight gain during pregnancy, the American Institute of Medicine17 guidelines are often used. (Box 3) The IOM defined suitable weight gain for women of varying BMI to reduce risk of perinatal mortality, premature birth and foetal growth retardation, to prevent foetal undernourishment and achieve optimal neonatal birth weight. The recommended weight gain is inversely related to pre-pregnancy BMI, but zero weight gain is not recommended.

An epidemiological study18 from the US suggested that obese women who gain less than 7kg during pregnancy have good outcomes and may fare even better in terms of pre-eclampsia, caesarean birth and excessive birth weight (although rates of small for gestational age were higher) than those gaining weight according to the IOM guidelines; furthermore, low weight gain has a weaker association with spontaneous preterm birth in obese women than in those with a lower BMI.¹⁹

However, obese women may not necessarily gain weight and may even lose weight if they adopt healthy dietary and physical activity patterns that they did not follow prior to conception. If dietary intake is adequate and foetal growth normal, these women can be reassured that their weight gain is appropriate.¹² Clearly more research is urgently needed to determine optimal pregnancy weight gain in obese women, particularly those with BMI over 40kg/m2.

A final consideration is for those women who have undergone bariatric surgery prior to pregnancy. Studies suggest that these women are less likely to experience complications than obese women who have not had the surgery.²⁰

MANAGEMENT OF OBESITY

NICE guidance recommends that healthcare professionals should make it clear to women with BMI over 30 the risks they face.¹¹ A first step is identifying whether a women is actually obese, and so measuring weight and height at the first contact, being sensitive to any concerns she may have regarding her weight, is crucial. From this, the BMI can easily be calculated. The BMI should be discussed with the patient and any risks explained, and a diet and physical activity plan agreed. Simple dietary goals might include eating breakfast, carrying healthy snacks and eating at least 5 portions of a variety of fruit and vegetables. The key points from the NICE guidance on weight management in pregnancy are shown in Box 4.

NICE guidance also recommends that obese mothers should be given practical advice and encouragement to lose weight before and after pregnancy, including access to specialist help if they need it.¹¹ Clearly there are great implications on already stretched resources to commit to such a plan.

For obese women, pregnancy is a window of opportunity to start to change towards healthier eating and an improved lifestyle, the benefits of which extend to the whole family, as well as healthier future pregnancies. In addition, weight retention and weight gain postpartum are important predictors of long-term obesity. Research suggests that appropriate diet and exercise during the post-natal period may help women to lose their postnatal weight.²¹

FUTURE HEALTH

In addition to the perinatal risks of obesity, some evidence suggests that a child of an obese mother may suffer from exposure to a nutritionally sub-optimal environment in utero. These early life adversities may extend decades later into adulthood, affecting development and future health. This seems to be especially true for some chronic diseases such as heart disease and diabetes. It is thought that there are critical periods during pregnancy and infancy where 'nutritional programming' occurs.²²

CONCLUSIONS

Pregnancy is a vulnerable time in terms of nutrition and health. The rising prevalence of obesity in the population of pregnant women could lead to increased risk of miscarriage, stillbirth and infant mortality. Achieving an optimal weight before pregnancy, and gaining appropriate amounts of weight during pregnancy is recommended to ensure a safe, healthy delivery and for the future health of the mother and child.

REFERENCES

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10. NICE Clinical Guideline 63. Diabetes in pregnancy: Management of diabetes and its complications from pre-conception to the postnatal period. NICE, London; 2008

11. NICE Public Health Guideline 27, 2010. Available at: http://guidance.nice.org.uk/PH27

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