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BMJ. 2007 July 28; 335(7612): 169.
PMCID: PMC1934487

Weight and pregnancy

Jennifer M Walsh, specialist registrar in obstetrics and gynaecology1 and Deirdre J Murphy, professor of obstetrics2

Women who maintain a normal healthy weight, before, during, and after pregnancy have better outcomes

Women of reproductive age are bombarded with messages about diet, weight, and body image. There is growing concern on the one hand about an epidemic of obesity, and on the other about a culture that promotes “size zero” as desirable, irrespective of a woman's natural build. Pregnancy is one of the most nutritionally demanding periods of a woman's life, with an adequate supply of nutrients essential to support fetal wellbeing and growth.1 With at least half of all pregnancies unplanned, women need to be aware of the implications of their weight for pregnancy, birth, and the health of their babies. However, the potential to provide women with conflicting information about weight, weight gain, and weight loss extends to pregnancy and birth outcomes.

Over one billion adults in the world are now overweight, with more than 300 million clinically obese.2 In the United States, the prevalence of obesity in women aged 20-39 years rose from 9% in 1960-1962 to 28% in 1999-2000.2 While weight and obesity have long concerned women in relation to body image and lifestyle matters, the association between weight gain and adverse pregnancy outcomes is less well recognised.

A recent nationwide Swedish cohort study involving 207 534 women from 1992 to 2001 examined the associations between changes in body mass index from the beginning of the first pregnancy to the beginning of the second in relation to adverse maternal and perinatal outcomes.3 Significantly increased rates of pre-eclampsia, gestational diabetes, pregnancy induced hypertension, and large for gestational age infants (odds of an adverse outcome were almost twofold in each case) were evident with increases in body mass index of just one to two units, and they increased progressively thereafter. An increase of greater than three body mass index units significantly increased the rate of term stillbirth, independent of obesity related diseases. Importantly, weight gain during the interval between pregnancies was strongly associated with major maternal and perinatal complications, independent of whether women were overweight (body mass index >25) by definition or not.

The key message is that women of normal weight should avoid gaining weight between pregnancies. In addition, overweight and obese women (body mass index ≥30) are likely to benefit from weight loss before becoming pregnant. However, while the authors have argued convincingly for a causal relation between maternal weight gain and adverse pregnancy outcomes, the advice given must be balanced to avoid weight swings in the opposite direction.

The association between low body mass index and subfertility is well known. Much less publicised is the association between low body mass index or substantial weight loss and pregnancy related complications, such as preterm birth and low infant birth weight. A second cohort study evaluated the impact of changing maternal nutritional status on the risk of prematurity, and specifically whether increasing or decreasing body mass index altered this risk.4 Overall, women whose body mass index fell by five or more units between pregnancies had a higher risk of preterm birth than women whose weight remained stable or who gained weight. The increased risk was particularly pronounced for women who had already experienced a preterm birth (80% v 28%). We should ensure that women of low body mass index attain a healthy weight before conception to reduce the risk of preterm birth and low infant birth weight. We should also counsel women with a history of previous preterm birth to maintain a healthy weight to prevent recurrence. In the context of the neonatal morbidity and mortality associated with preterm birth, low body mass index is one of the few modifiable risk factors.

The challenge for healthcare professionals is in interpreting these findings and advising women accordingly. Women are at increased risk of different but equally serious adverse pregnancy outcomes if they gain or lose an excessive amount of weight between pregnancies. Although apparently conflicting, these studies show how important it is to attain and maintain a normal healthy weight before, during, and after pregnancy. Most women wish to achieve the best start in life for their babies. This powerful motivation could be used to achieve behavioural changes in terms of a healthy balanced diet and maintaining a stable weight within the normal range for the woman's build. This approach offers long term health benefits for women and their babies. Certainly, any woman who has had a poor obstetric outcome should be encouraged to achieve an optimal weight before planning another pregnancy. The challenge for many women of achieving a stable body mass index cannot be underestimated, and this may require professional support and advice.

Notes

Competing interests: None declared.

Provenance and peer review: Commissioned; not externally peer reviewed.

References

1. International Food Information Council Foundation. Healthy eating during pregnancy. 2003. http://ific.org/publications/brochures/pregnancybroch.cfm?renderforprint=1.
2. WHO. Obesity: preventing and managing the global epidemic. London: Stationery Office Books, 2000. Publication WHO/NUT/NCT/98.1.
3. Villamor E, Cnattingus S. Interpregnancy weight change and risk of adverse pregnancy outcomes: a population based study. Lancet 2006;368:1164-70. [PubMed]
4. Merlino A, Laffineuse L, Collin M, Mercer B. Impact of weight loss between pregnancies on recurrent preterm birth. Am J Obstet Gynecol 2006;195:818-21. [PubMed]

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