Association among pre-pregnancy body mass index, gestational weight gain and neonatal birth weight: a prospective cohort study in China

Since the 1980s, the overweight and obesity rates have doubled during the past four decades in more than 70 countries worldwide, and the rates are still increasing [1]. The same trend have been observed among Chinese adults: the overweight and obesity rates among this population were 20 and 7.1%, respectively, according to a national survey conducted in 2002 [2]. For women of childbearing age, pre-pregnancy body mass index (BMI) is regarded as an important predictor of pregnancy outcomes since abnormal BMI has been verified to be related to a series of adverse maternal and neonatal complications [3‐5]. However, the incidence of pre-pregnancy overweight and obesity has increased significantly around the world and has become an urgent public health problem. Gestational weight gain (GWG) is another factor that impacts fetal development, and insufficient or excessive GWG mostly results in restricted or over development. Asian women have been shown to have the highest prevalence of insufficient GWG (USA 21%, Europe 18% and Asia 31%), while more than half of American women have excessive GWG (USA 51%, Europe 51% and Asia 31%) [6].

Although GWG is commonly used to evaluate fetal development, it has some limitations. Weight gain has different effects on fetal development in different trimesters, and total GWG failed to show trimester-specific weight change during pregnancy. A large number of clinical studies have demonstrated that women’s weight gain during the 2nd and 3rd trimester (GWG_rate), which is also the period of maximal growth and weight gain for fetuses, is significantly associated with newborn weight while weight gain in the 1st trimester mostly affects overweight or obesity risk in the offspring’s childhood [7]. Thus, GWG_rate is preferable for exploring the relationship between trimester-specific weight change and infant birth weight even though the data collection is complicated. The overall prevalence of insufficient and excessive GWG_rate varies in different regions [8, 9]. Some studies showed that 12.5 and 57.9% of Chinese women had insufficient and excessive GWG_rate respectively in 2013 [10], but the updated incidence rate is unclear.

Most studies have shown that pre-pregnancy underweight and insufficient GWG are associated with a higher risk of having small for gestational age (SGA) infants but a lower risk of delivering large for gestational age (LGA) infants [11]. Similarly, pre-pregnancy overweight/obese and excessive GWG are associated with higher LGA risk but lower SGA risk despite of few studies that have inconsistent results [12‐14]. However, some research indicates that the relationship between pre-pregnancy BMI, GWG and infant birth weight varies among different races of women, and whether the relationship is the same among Chinese women is not clear [15]. In addition, most research in China is province-based or city-based and lacks representativeness. As for the association between GWG_rate and birth weight, relevant studies are even mere, and the results are also inconsistent [8, 16]. Furthermore, few studies have explored the dose-response relationship between women’s body weight before or during pregnancy and infant birth weight. Hence, conducting more related studies and making the results more comprehensive is meaningful.

This study aimed to evaluate the prevalence of pre-pregnancy BMI, GWG and GWG_rate in the 2nd and 3rd trimester, and to explore their associations with abnormal neonatal birth weight (i.e. SGA and LGA) among Chinese women, as well as the dose-response relationship between women’s weight status before or during pregnancy and abnormal birth weight risks.

A total of 3585 women were enrolled and maternal characteristics presented by neonatal birth weight are shown in Table 1. The overall prevalence of SGA and LGA was 5.77 and 10.54%, respectively, among these respondents, and the average birth weight was 3321 g (SD: 453 g). Women who were younger and employed were more likely to have SGA infants, while older and unemployed women tended to have LGA infants (P < 0.05). SGA mothers were shown to have fewer gravidities and parities than LGA mothers. In addition, women who gave birth to SGA infants were less likely to have GDM but had higher risk of gestational hypertension compared with women in the LGA group (all P < 0.05).

Table S2 to Table S4 [see Additional file] show the demographic and clinical characteristics of the study participants by pre-pregnancy BMI, GWG and GWG_rate categories. The prevalence of underweight, overweight and obesity before pregnancy was 14.17, 12.86 and 1.73%, respectively. According to the IOM recommendation, 24.69 and 33.78% of women had insufficient and excessive GWG, respectively. Table S4 [see Additional file] illustrates the prevalence of SGA and LAG by pre-pregnancy, GWG and GWG_rate categories. Approximately 26.86% of women showed insufficient GWG_rate while 46.95% had excessive GWG_rate. The average birth weight of infants was associated with women’s pre-pregnancy BMI, GWG and GWG_rate (P < 0.001).

The multinomial logistic regression analysis results are shown in Table 2. Women who were underweight before pregnancy, had insufficient GWG or had insufficient GWG_rate were 1.9-,1,9- and 1.5-fold more likely to have SGA infants, respectively, compared with reference groups, and women who were overweight or obese before pregnancy, had excessive GWG or had excessive GWG_rate were 2.0-,1.7- and 1.7-fold more likely to have LGA infants, respectively. Additionally, protective effects were shown between excessive GWG_rate and SGA, as well as pre-pregnancy underweight, insufficient GWG and LGA.

Chi-square analysis demonstrated the relationship between birth weight and jointed pre-pregnancy BMI and GWG/GWG_rate [see Table S5 to Table S7 in Additional file]. The prevalence of insufficient GWG in the pre-pregnancy underweight, normal weight and overweight/obesity groups was 28.54, 27.21 and 8.60%, respectively, and the prevalence of excessive GWG was 24.41, 30.58 and 58.51%, respectively, in each group. GWG and GWG_rate were associated with infant birth weight under different pre-pregnancy BMI levels (P < 0.05).

Stratified analysis results are shown in Table 3. Among the pre-pregnancy underweight, normal weight and overweight/obese groups, women with insufficient GWG were 3.0-, 1.2- and 6.7-fold more likely to have SGA infants, respectively. Excessive GWG among normal weight and overweight/obese women was 1.6- and 2.3-fold more likely to have LGA infants, respectively. Only excessive GWG among the normal weight group had a protective effect on SGA.

The association of neonatal birth weight with GWG_rate under each pre-pregnancy BMI level is shown in Table 4. For women with normal BMI before pregnancy, excessive GWG_rate was associated with a 1.6 times higher risk for LGA but a 0.35 times lower risk for SGA. Excessive GWG_rate also contributed to a 1.7 times higher risk for LGA among pre-pregnancy overweight and obese women, and insufficient GWG_rate was associated with a 0.81 times lower LGA risk in this group.

Figure 1 shows dose-response relationships between neonatal birth weight and women’s body weight before and during pregnancy. The RCS logistic regression models showed that except for the nonlinear association between GWG_rate and LGA (P_nonlinear < 0.001), the other associations were linear (P_nonlinear > 0.05).

In this study, we found that pre-pregnancy underweight and insufficient GWG were associated with higher SGA risk and lower LGA risk while pre-pregnancy overweight or obese and excessive GWG were associated with higher LGA risk and lower SGA risk. GWG_rate was also associated with neonatal birthweight. The association of GWG and GWG_rate with birth weight varies with pre-pregnancy BMI status. Dose-response relationships existed between pre-pregnancy BMI, GWG, GWG_rate and newborn weight.

The relationships this study found between pre-pregnancy BMI, GWG and GWG_rate are consistent with the findings of some existing studies [21‐23], including a cohort study located in three provinces in southwest China [24]. A meta-analysis showed that insufficient GWG contributed to a lower risk for LGA while excessive GWG was associated with lower risk for SGA [11]. We also found that insufficient GWG was associated with lower LGA, but this relationship disappeared when we adjusted pre-pregnancy BMI. Xie [25] found that excessive GWG increased neonatal birth weight, but no relationship was found between pre-pregnancy BMI and birth weight. However, Ratnasiri [26] conducted a retrospective cohort study, and the results showed pre-pregnancy overweight and obese contributed to a lower SGA rate and pre-pregnancy underweight decreased the risk of LGA. These differences may be attributed to ethnicity. As for GWG_rate, another Chinese cohort study indicated that excessive GWG_rate increased LGA risk, but insufficient GWG_rate had no impact on SGA, and this result was inconsistent with our finding [27]. Differences in findings may be due to different study population since this study was conducted in 15 different provinces and the sample could better represent Chinese women. The research mentioned above mostly focuses on specific areas of China or foreign countries, and various cultures, lifestyles, economic statuses and many other factors may contribute to the differences in the results.

The association of GWG and GWG_rate and birth weight varies with different pre-pregnancy BMI statuses. Stratified analysis showed that insufficient GWG increased SGA risk only in pre-pregnancy underweight or normal weight women, rather than overweight or obese women. This finding is consistent with a cohort study targeted at US women [28]. This study also found that excessive GWG decreased SGA risk only in women with a normal body weight before pregnancy. However, this finding is inconsistent with a retrospective cohort study conducted by Li [29], which showed that excessive GWG had a protective effect on SGA in pre-pregnancy overweight and obese women. We also did not find that GWG made any effort towards LGA in pre-pregnancy underweight women. Excessive GWG_rate was associated with a higher risk of LGA except in pre-pregnancy underweight women, and it was found to decrease SGA risk only in pre-pregnancy normal weight. However, some research [30] found that insufficient GWG_rate could decrease LGA risk in pre-pregnancy underweight women, but we did not find the same results. This study also did not show relationship between insufficient GWG_rate and the risk of SGA, which are commonly believed to be associated. This difference indicates that more comprehensive and well-designed studies are needed to help determining consistent results.

There are relatively few studies focusing on the dose-response relationship between women’s body weight before and during pregnancy and infant birth weight. Consistent results can be found through these studies that a dose-response relationship exists, but the shapes of these curves are different [31, 32]. Except for the relationship between GWG_rate and LGA, which showed an ‘S-shaped’ carve, others were ‘L-shaped’ in this study. These results were different from those of the previously mentioned studies, which may be attributed to different research populations or study designs. Nevertheless, dose-response research on this topic is mere, and far more studies are needed.

The novelty of this study is significant. First, this study explored the protective effects of pre-pregnancy underweight/ insufficient GWG and GWG_rate on LGA infants among Chinese women and the protective effects of pre-pregnancy overweight and obese/ excessive GWG and GWG_rate on SGA infants, which are still inconsistent yet. Second, stratified analysis was adopted to explore the relationships between GWG/GWG_rate and birth weight under different pre-pregnancy BMI levels, and this made our study more comprehensive and accurate; Third, this study analyzed GWG_rate, which was less commonly researched due to cumbersome measurement, and the results can provide references for further relevant research. Fourth, we explored the dose-relationships between women’s body weight before and during pregnancy and neonatal birth weight, and this was focused by only few studies around the word.

This study has some strengths and limitations. This nationwide multicenter prospective cohort study enrolled people living in 15 different provinces in China, and this sample can well represent Chinese women. Core data, including weight before pregnancy and weight at delivery, gestational age and neonatal birth weight, were double checked to maintain data quality, which means data collected by questionnaire were checked with that recorded in the HIS system of each hospital. If there was any inconsistency, a medical record was preferred unless it was illogical. For limitations, respondents were asked to recall their pre-pregnancy body weight and height, which may result in recall bias. In addition, convenience sampling may decrease the representativeness of our results. We did not conduct subgroup analysis stratified by obesity class due to the small sample size of pre-pregnancy obese women. However, this subgroup analysis may provide more detailed references for body weight control before pregnancy and it should be explored in the future.

In summary, this study showed that pre-pregnancy underweight, insufficient GWG and GWG_rate increased the risk of SGA while pre-pregnancy overweight or obese, excessive GWG and GWG_rate increased the risk of LGA. In addition, these relationships varied according to different pre-pregnancy BMI statuses. Dose-response relationships were observed between independent and dependent variates mentioned above. Our results emphasized the significance of body weight control both before and during pregnancy. Although it may be difficult for women to adhere the IOM recommendation, interventions such as education should be imposed to help achieving suitable pre-pregnancy BMI and GWG [33].