Intergenerational transmission of birth weight: a systematic review and meta-analysis

The objectives of this study were (1) to systematically review the literature on the association between birth weight in children born in the first and second generation and (2) to quantify this association by performing a meta-analysis. A systematic review was carried out in six databases (PubMed, Science Direct, Web of Science, Embase, Scopus, CINAHL and LILACS), in January 2021, for studies that recorded the birth weight of parents and children. A meta-analysis using random effects to obtain a pooled effect of the difference in birth weight and the association of low birth weight (LBW) between generations was performed. Furthermore, univariable meta-regression was conducted to assess heterogeneity. Egger's tests were used to possible publication biases. Of the 9878 identified studies, seventy were read in full and twenty were included in the meta-analysis (ten prospective cohorts and ten retrospective cohorts), fourteen studies for difference in means and eleven studies for the association of LBW between generations (twenty-three estimates). Across all studies, there was no statistically significant mean difference (MD) birth weight between first and second generation (MD 19·26, 95 % CI 28·85, 67·36; P = 0·43). Overall, children of LBW parents were 69 % more likely to have LBW (pooled effect size 1·69, 95 % CI (1·46, 1·95); I2:85·8 %). No source of heterogeneity was identified among the studies and no publication bias. The average birth weight of parents does not influence the average birth weight of children; however, the proportion of LBW among the parents seems to affect the offspring's birth weight.

Does Maternal Country of Birth Matter for Understanding Offspring's Birthweight? A Multilevel Analysis of Individual Heterogeneity in Sweden

Background

Many public health and epidemiological studies have found differences between populations (e.g. maternal countries of birth) in average values of a health indicator (e.g. mean offspring birthweight). However, the approach based solely on population-level averages compromises our understanding of variability in individuals’ health around the averages. If this variability is high, the exclusive study of averages may give misleading information. This idea is relevant when investigating country of birth differences in health.

Methods and Results

To exemplify this concept, we use information from the Swedish Medical Birth Register (2002–2010) and apply multilevel regression analysis of birthweight, with babies (n = 811,329) at the first, mothers (n = 571,876) at the second, and maternal countries of birth (n = 109) at the third level. We disentangle offspring, maternal and maternal country of birth components of the total offspring heterogeneity in birthweight for babies born within the normal timespan (37–42 weeks). We found that of such birthweight variation about 50% was at the baby level, 47% at the maternal level and only 3% at the maternal countries of birth level.

Conclusion

In spite of seemingly large differences in average birthweight among maternal countries of birth (range 3290–3677g), knowledge of the maternal country of birth does not provide accurate information for ascertaining individual offspring birthweight because of the high inter-offspring heterogeneity around country averages. Our study exemplifies the need for a better understanding of individual health diversity for which group averages may provide insufficient and even misleading information. The analytical approach we outline is therefore relevant to investigations of country of birth (and ethnic) differences in health in general.

Revisiting the effect of maternal smoking during pregnancy on offspring birthweight: a quasi-experimental sibling analysis in Sweden

Maternal smoking during pregnancy (SDP) seems associated with reduced birthweight in the offspring. This observation, however, is based on conventional epidemiological analyses, and it might be confounded by unobserved maternal characteristics related to both smoking habits and offspring birth weight. Therefore, we apply a quasi-experimental sibling analysis to revisit previous findings. Using the Swedish Medical Birth Register, we identified 677,922 singletons born between 2002 and 2010 from native Swedish mothers. From this population, we isolated 62,941 siblings from 28,768 mothers with discrepant habits of SDP. We applied conventional and mother-specific multilevel linear regression models to investigate the association between maternal SDP and offspring birthweight. Depending on the mother was light or heavy smoker and the timing of exposition during pregnancy (i.e., first or third trimester), the effect of smoking on birthweight reduction was between 6 and 78 g less marked in the sibling analysis than in the conventional analysis. Sibling analysis showed that continuous smoking reduces birthweight by 162 grams for mothers who were light smokers (1 to 9 cigarettes per day) and 226 g on average for those who were heavy smokers throughout the pregnancy in comparison to non-smoker mothers. Quitting smoking during pregnancy partly counteracted the smoking-related birthweight reduction by 1 to 29 g, and a subsequent smoking relapse during pregnancy reduced birthweight by 77 to 83 g. The sibling analysis provides strong evidence that maternal SDP reduces offspring birthweight, though this reduction was not as great as that observed in the conventional analysis. Our findings support public health interventions aimed to prevent SDP and to persuade those who already smoke to quit and not relapse throughout the pregnancy. Besides, further analyses are needed in order to explain the mechanisms through which smoking reduces birthweight and to identify other maternal characteristics that are common causes of both birthweight reduction and maternal smoking.