Previous studies have suggested that maternal active smoking can increase the risk of birth defects, but evidence on second-hand tobacco smoke (SHS) is limited. We aimed to assess the association between maternal exposure to SHS and birth defects in a Chinese population. The data were based on a large-scale cross-sectional survey conducted in Shaanxi Province, China. Considering the characteristics of survey design and the potential impact of confounding factors, we adopted propensity score matching (PSM) to match the SHS exposure group and the non-exposure group to attain a balance of the confounders between the two groups. Subsequently, conditional logistic regression was employed to estimate the effect of SHS exposure on birth defects. Furthermore, sensitivity analyses were conducted to verify the key findings. After nearest neighbor matching of PSM with a ratio of 2 and a caliper width of 0.03, there were 6,205 and 12,410 participants in the exposure and control group, respectively. Pregnant women exposed to SHS were estimated to be 58% more likely to have infants with overall birth defects (OR = 1.58, 95% CI: 1.30–1.91) and 75% more likely to have infants with circulatory system defects (OR = 1.75, 95% CI: 1.26–2.44). We also observed that the risk effect of overall birth defects had an increasing trend as the frequency of exposure increased. Additionally, sensitivity analyses suggested that our results had good robustness. These results indicate that maternal exposure to SHS likely increases the risk of overall birth defects, especially circulatory system defects, in Chinese offspring.

Despite high altitude was implicated in adverse birth outcomes, there remained a paucity of evidence on low-to-medium altitude effect. This study aimed to explore the association of low-to-medium altitude with birth outcomes. A population-based cross-sectional survey was performed using a stratified multistage random sampling method among women with their infants born during 2010–2013 in Northwestern China. Altitude was determined in meters based on the village or community of the mother’s living areas. Birth outcomes involved birth weight, gestational age, and small for gestational age (SGA). Generalized linear models were fitted to investigate the association of altitude with birth outcomes. Moreover, the dose–response relationship between altitude and birth outcomes was evaluated with a restricted cubic spline function. A total of 27 801 women with their infants were included. After adjusting for potential confounders, every 100-m increase in the altitude was associated with reduced birth weight by 6.4 (95% CI −8.1, −4.6) g, the slight increase of gestational age by 0.015 (95% CI 0.010, 0.020) week, and an increased risk of SGA birth (odds ratio 1.03, 95% CI 1.02, 1.04). Moreover, there was an inversely linear relationship between altitude and birth weight (P for overall < 0.001 and P for nonlinear = 0.312), and a positive linear relationship between altitude and SGA (P for overall < 0.001 and P for nonlinear = 0.194). However, a nonlinear relationship was observed between altitude and gestational age (P for overall < 0.001 and P for nonlinear = 0.010). The present results suggest that low-to-medium altitude is possibly associated with adverse birth outcomes.

Few studies have investigated the association between maternal dietary patterns (DP) during pregnancy, derived from reduced-rank regression (RRR), and fetal growth. This study aims to identify DP during pregnancy associated with macro- and micronutrient intakes, using the RRR method, and to examine their relationship with birth weight (BW). We used data of 7194 women from a large-scale cross-sectional survey in Northwest China. Dietary protein, carbohydrate, haem Fe density and the ratio of PUFA and MUFA:SFA were used as the intermediate variables in the RRR model to extract DP. Generalised estimating equation models were applied to evaluate the associations between DP and BW and related outcomes (including BW z-score, low birth weight (LBW) and small for gestational age (SGA)). Four DP during pregnancy were identified. Socio-demographically disadvantaged pregnant women were more likely to have lower BW and lower adherence to DP1 (high legumes, soyabean products, vegetables and animal-source foods, with relative low wheat and oils). Women with medium and high adherence to DP1 had significantly increased BW (medium 28·6 (95 % CI 7·1, 50·1); high 25·2 (95 % CI 2·7, 47·6)) and BW z-score and had significantly reduced risks of LBW and SGA. The associations were stronger among women with babies <3100 g. There is no association between other DP and outcomes. Higher adherence to the DP that was high in legumes, soyabean products, vegetables and animal-source foods was associated with improved BW in the Chinese pregnant women, particularly among those with disadvantageous socio-demographic conditions.

The effect of maternal folate intake on small-for-gestational-age (SGA) births remains inconclusive. The present study aimed to investigate the associations of maternal folate intake from diet and supplements with the risk of SGA births using data from a cross-sectional study in Shaanxi Province of Northwest China. A total of 7307 women who were within 12 months (median 3; 10th–90th percentile 0–7) after delivery were included. Two-level models were adopted to examine the associations of folate (dietary folate, supplemental folic acid and total folate) intake with the risk of SGA births and birth weight Z score, controlling for a minimum set of confounders that were identified in a directed acyclic graph. Results showed that a higher supplemental folic acid intake during the first trimester was negatively associated with the risk of SGA births (≤60 d v. non-use: OR 0·80; 95 % CI 0·66, 0·96; >60 d v. non-use: OR 0·78; 95 % CI 0·65, 0·94; Ptrend = 0·010; per 10-d increase: OR 0·97; 95 % CI 0·95, 0·99). A higher total folate intake during pregnancy was associated with a reduced risk of SGA births (highest tertile v. lowest tertile: OR 0·77; 95 % CI 0·64, 0·94; Ptrend = 0·010; per one-unit increase in the log-transformed value: OR 0·81; 95 % CI 0·69, 0·95). A similar pattern was observed for the birth weight Z score. Our study suggested that folic acid supplementation during the first trimester and a higher total folate intake during pregnancy were associated with a reduced risk of SGA births.

Anaemia during pregnancy, characterised by Hb <110 g/l, is a specific risk factor for adverse maternal and perinatal outcomes in developing countries. The objective of this study was to determine the effectiveness of daily antenatal supplementation with multiple micronutrients (MMN) compared with folic acid (FA) on the occurrence of anaemia among pregnant women and their infants’ health in a high-altitude area. A prospective cohort study was carried out in two rural counties in Tibet from 2007 to 2012. A total of 1149 eligible pregnant women were allocated daily supplementation with FA in one county and MMN containing a recommended allowance of twenty-three vitamins and minerals in another county starting ≤24 weeks of gestation and continuing until delivery. Compared with the FA group, prenatal supplementation with MMN was significantly associated with reduced odds of anaemia in the third trimester. This was demonstrated in the primary outcome, with an adjusted OR (AOR) of 0·63; 95 % CI 0·45, 0·88 and P=0·007 and also reduced odds of preterm delivery (AOR: 0·31; 95 % CI 0·15, 0·61; P=0·001). There was no difference between MMN and FA groups in mean birth weight (adjusted mean difference: 36·78; 95 % CI −19·42, 92·98 g; P=0·200), whereas MMN supplementation significantly reduced the odds of low-birth weight (LBW) babies (AOR: 0·58; 95 % CI 0·36, 0·91; P=0·019). In conclusion, the antenatal MMN supplementation in rural Tibet is associated with a reduction of maternal anaemia in the third trimester, and may potentially decrease the risk of preterm delivery and LBW babies.

Previous studies have yielded conflicting results on the associations of maternal Fe intake with birth outcomes. This study aimed to investigate the associations between maternal Fe intake (total Fe from diet and supplements, dietary total Fe, haeme Fe, non-haeme Fe and Fe supplements use) and adverse birth outcomes in Shaanxi Province of Northwest China. In all, 7375 women were recruited using a stratified multistage random sampling method at 0–12 months (median 3; 10th–90th percentile 0–7) after delivery. Diets were collected by a validated FFQ and maternal characteristics were obtained via a standard questionnaire. The highest tertile of haeme Fe intake compared with the lowest tertile was negatively associated with low birth weight (LBW) (OR 0·68; 95 % CI 0·49, 0·94), small for gestational age (SGA) (OR 0·76; 95 % CI 0·62, 0·94) and birth defects (OR 0·55; 95 % CI 0·32, 0·89). Maternal haeme Fe intake was associated with a lower risk of intra-uterine growth retardation (IUGR) (medium tertile v. lowest tertile: OR 0·78; 95 % CI 0·61, 0·95; highest tertile v. lowest tertile: OR 0·76; 95 % CI 0·59, 0·93; Ptrend=0·045). The OR of LBW associated with Fe supplements use were as follows: during pregnancy: 0·72 (95 % CI 0·50, 0·95); in the second trimester: 0·67 (95 % CI 0·42, 0·98); in the third trimester: 0·47 (95 % CI 0·24, 0·93). We observed no associations of total Fe, dietary total Fe or non-haeme Fe intake with birth outcomes. The results suggest that maternal haeme Fe intake is associated with a reduced risk of LBW, SGA, IUGR and birth defects, and Fe supplements use during pregnancy reduces LBW risk.