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If a woman is heavy when she becomes pregnant, it can affect both her and her child's health. The health risk is larger if she gains excessive weight during the pregnancy. The advice to women with body mass index of 30 or above (obesity) is that they should gain 5-9 kg while pregnant. Research suggests that even lower gestational weight gain or a small weight loss is beneficial for these women. The question is whether it is safe for the child.
We found four studies where pregnant women with obesity in the intervention group received additional care to achieve less than 5 kg gestational weight gain. Even though the women in the intervention group received substantial support in changing the diet and increasing their physical activity, not all achieved the goal of low gestational weight gain. The studies compared with women who received regular antenatal care.
Few of the women miscarried or had preterm births (before pregnancy week 37). Among the 1160 women who either attempted to achieve low gestational weight gain or received regular antenatal care, three stillbirths were registered in total. Only two of the four studies reported the number of children with low birth weight. The studies gave uncertain estimates for the effect on the proportion of children with high birth weight.
The findings are too uncertain to judge whether support to achieve low gestational weight gain changes the risk of miscarriage, preterm birth, stillbirth and low birth weight.
Obesity in pregnant women and large weight gain during pregnancy increase the risk of several harmful outcomes for the mother and the child. The Institute of Medicine (IOM) in the US recommended in 2009 that women with obesity, defined as pre-pregnancy body mass index (BMI) ≥ 30, should aim at 5-9 kg gestational weight gain. Such limited weight gain will be beneficial for both the mother’s and the child’s health. However, this recommendation does not differentiate between different degrees of obesity. Increased degree of maternal obesity is associated with higher risk of the adverse health outcomes. With regard to the women's health, especially those with pre-pregnant BMI ≥ 35, it will be beneficial with even lower gestational weight gain or a weight loss. However, it is unclear whether this is safe for the child. Risks associated with low gestational weight gain or weight loss can be increased chance of stillbirth, premature birth and low birth weight.
This report is a systematic review of the effects of lifestyle interventions aiming for low weight gain (< 5 kg) or weight loss for pregnant women with pre-pregnant BMI ≥ 30.
We searched for primary studies in Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, CINAHL, PsycInfo and Swemed+ until November 2017 and checked reference lists in included studies. Two persons independently examined 6851 references, evaluated 37 of these in full text and included four studies. The study population was pregnant women with pre-pregnant body mass index ≥ 30. The intervention was measures to change lifestyle habits (primarily diet and/or physical activity) aiming for low gestational weight gain (< 5 kg) or weight loss, in addition to standard antenatal care. The comparison condition could be: 1) measures aiming at 5-9 kg gestational weight gain; or 2) treatment described as regular or other antenatal care for the target group. We searched for randomized, controlled studies or other controlled designs. Two people independently assessed risk of bias in each included study. One person retrieved data from the studies, conducted meta-analyses and assessed our confidence in the documentation of effect, and another person checked this. We used the GRADE methodology (Grading of Recommendations Assessment, Development and Evaluation) to assess our confidence in the estimated effects. Our confidence can be high, moderate, low or very low. If sufficient data were available, it was planned to do separate analyses or compare the effects for pregnant women with pre-pregnant obesity grade 1 (BMI 30-34.9), grade 2 (BMI 35-39.9) or grade 3 (BMI 40 or more). None of the studies presented the results in such a way that separate analyses were possible.
We included four randomized, controlled trials, two from Denmark and two from the US. In total, 1160 pregnant women with pre-pregnant BMI ≥ 30 participated. The two American studies had more participants with obesity grade 2 and 3 than the two Danish studies. All studies examined the effect of measures to achieve < 5 kg or no weight gain in pregnancy, but one study also had a small weight loss as possible treatment goals. The women in the intervention group received several activities in addition to regular antenatal care. All received individual, repeated dietary counselling focusing on dietary habits and controlled energy intake, as well as advice on increased physical activity. In some studies, they were offered fitness classes, as well as counselling, individually or in groups, to increase motivation to change lifestyle habits. Although the studies provided slightly different measures, we considered that the results could be analysed as one intervention in meta-analysis of effect.
None of the studies compared the intervention with measures to achieve 5-9 kg gestational weight gain. The comparison condition in all studies were presented as regular antenatal care at the study clinics. This included some advice on healthy diet, physical activity and limited weight gain in pregnancy. In one study, women in the control group were told that gestational weight gain < 5 kg was desirable, but they received no activities or further support to achieve this target. In three of the four studies had gestational weight gain as their primary outcome, not outcomes related to the child.
Even though the women in the intervention groups received considerable support, not all achieved the weight goals. The mean or median weight gain among pregnant women in the intervention groups ranged from 5.0 kg to 9.4 kg, respectively, in the four studies. In the two Danish studies, approximately 1/4 of the women in the intervention groups achieved < 5 kg gestational weight gain, and so did approximately 1/5 of the women in the control groups. The difference in weight change between the groups was somewhat larger in the two American studies. The intervention groups had lower gestational weight gain compared to the control group in all studies, but the difference was limited - down to less than 2 kg difference in median weight change.
When comparing the effects of measures designed for women with pre-pregnant BMI ≥ 30 to achieve < 5 kg gestational weight gain or a weight loss (the intervention) with regular antenatal care for pregnant women with BMI ≥ 30 where the study was conducted (the control condition), we found that:
- Few of the women miscarried or had premature births (before pregnancy week 37). Stillbirth was rare and occurred to three of the 1160 women who participated in these studies. The findings are therefore too uncertain to assess whether the intervention affect the risk of miscarriage (Relative risk (RR) 0.58 (95% confidence interval (CI): 0.11 to 2.98)), premature birth (RR 1.13 (95% CI: 0.54 to 2.38)) and stillbirth (Odds ratio 1.60 (95% CI: 0.08 to 94.8)).
- The intervention may have little or no effect on the prevalence of large for gestational age (RR 0.82 (95% CI: 0.4 to 1.72)) or macrosomic infants (RR 1.01 (95% CI: 0.71 to 1.44)) compared to the control condition, but the error margin for the effect estimate includes both an increase and reduction in risk.
- The findings are too uncertain to assess whether the intervention affects the risk of low birth weight/small for gestational age (RR 1.48 (95% CI: 0.4 to 5.44)) and overweight among the children after 2½ to 3 years (RR 1.72 (95% CI: 0.6 to 4.85)).
- The intervention may have little or no effect on the incidence of gestational diabetes (RR 1.16 (95% CI: 0.75 to 1.8)) and pre-eclampsia (RR 0.83 (95% CI: 0.56 to 1.23)) compared to the control condition, but the error margin for the effect estimate includes both an increase and reduction in risk.
- The intervention shows a trend towards a small reduction in the risk of caesarean section (RR 0.91 (95% CI: 0.81 to 1.03)) compared to the control condition, but little or no effect is also likely.
The main question for this systematic overview is whether the risk of adverse outcomes for the child increases if women with obesity achieves < 5 kg gestational weight gain or a weight loss. The research question in three of the four included studies, however, was whether additional measures were effective in achieving < 5 kg gestational weight gain in women with obesity. A significant proportion of the women in the intervention groups did not achieve the weight goal. At the same time, some pregnant women in the control group achieved the low weight gain target. This may be due to the characteristics of antenatal care they received or that both the study personnel and the women knew the purpose of the study. The difference in weight gain between the groups is relatively low. It is an important principle for randomized controlled studies that participants are analysed by their distributed group, regardless of the treatment they received. Participants probably received the assigned measures from the clinics, but the intervention required significant engagement from the women themselves to change diet and other living habits.
Finding effective measures for lifestyle changes and weight control among pregnant women, especially pregnant with obesity, is a challenge. Earlier evidence summaries show that it is also difficult for women to achieve IOM's gestational weight recommendations. However, since it is unclear whether < 5 kg gestational weight gain or a weight loss for obese pregnant women can be harmful to the child, it is important that such studies focus on possible adverse events. It is conspicuous that two of the studies do not indicate the proportion of children with low birth weights despite presenting the proportion of high birth weight. Miscarriage after randomization is treated as loss to follow up, not an outcome, in all the included studies. Stillbirths were probably less common in the studies than in the countries where the studies were conducted and in Norway (3.5 deaths per 1,000 pregnant women in 2016). This may be because women with high-risk pregnancies were not included in the studies.
The four included studies comprised in total 1160 pregnant women. However, due to rare outcomes and missing data reporting, these studies have, also analysed together, too few participants to provide well-founded conclusions.
There is too little information to assess whether measures to achieve < 5 kg gestational weight gain or a weight loss for pregnant women with BMI ≥ 30 changes the risk of miscarriage, preterm birth, stillbirth and low birth weight.