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Background: Advanced maternal age at birth is considered a risk factor for adverse birth outcomes. A recent study applying a sibling design has shown, however, that the association might be confounded by unobserved maternal characteristics.Methods: Using total population register data on all live singleton births during the period 1999-2012 in Denmark (N = 580 133; 90% population coverage), Norway (N = 540 890) and Sweden (N = 941 403) and from 2001-2014 in Finland (N = 568 026), we test whether advanced maternal age at birth independently increases the risk of low birthweight (LBW) (<2500 g) and pre-term birth (<37 weeks gestation). We estimated within-family models to reduce confounding by unobserved maternal characteristics shared by siblings using three model specifications: Model 0 examines the bivariate association; Model 1 adjusts for parity and sex; Model 2 for parity, sex and birth year.Results: The main results (Model 1) show an increased risk in LBW and pre-term delivery with increasing maternal ages. For example, compared with maternal ages of 26-27 years, maternal ages of 38-39 years display a 2.2, 0.9, 2.1 and 2.4 percentage point increase in the risk of LBW in Denmark, Finland, Norway and Sweden, respectively. The same patterns hold for pre-term delivery.Conclusions: Advanced maternal age is independently associated with higher risk of poor perinatal health outcomes even after adjusting for all observed and unobserved factors shared between siblings. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the association between sibling relatedness and pubertal development in girls and boys., Methods: This cohort study consisted of 10,657 children from the Puberty Cohort, Denmark. Information on sibling relatedness was obtained by self-report. Information on pubertal markers was obtained half yearly from age 11 and throughout puberty. Mean age difference at attaining pubertal markers was estimated using interval-censored regression models according to sibling relatedness (full, half and/or step siblings; half and/or step siblings; no siblings; relative to full siblings)., Results: Girls with both full, half and/or step siblings (-1.2 (CI 95 %: -2.5; 0.1) months), only half- and/or stepsiblings (-2.2 (CI 95 %: -3.7; -0.7) months), and no siblings (-5.5 (CI 95 %: -8.5; -2.5) months) entered puberty earlier than girls with full siblings. Boys with full, half and/or step siblings (-1.4 (CI 95 %: -2.7; -0.1) months), only half and/or step siblings (-1.2 (CI 95 %: -3.0; 0.6) months), and no siblings (-4.5 (CI 95 %: -8.8; -0.3) months) entered puberty earlier than boys with full siblings., Conclusions: Children with sibling relatedness other than full siblings entered puberty earlier than their peers with full siblings even after adjustment for parental cohabitation status, childhood body mass index and childhood internalizing and externalizing symptoms., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Cecilia H. Ramlau-Hansen, Siri Eldevik Haberg, Mikko Myrskyla reports financial support was provided by European Research Council. Cecilia Ramlau-Hansen reports financial support was provided by Independent Research Fund Denmark. Cecilia H. Ramlau-Hansen reports financial support was provided by Independent Research Fund Denmark Health and Disease. Mikko Myrskyla reports financial support was provided by Strategic Research Council. Mikko Myrskyla reports financial support was provided by National Institute on Aging. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Disclosure of interest All authors declare no conflicts of interest. CHR declares to have received funding from the Danish Council for Independent Research (DFF 4183–00152), the Independent Research Fund Denmark (FSS 0602–02738B), the European Union (ERC, BIOSFER, 101071773). SEH declares to have received funding from the Research Council of Norway through its Centres of Excellence funding scheme (project No 262700), the European Union (ERC, BIOSFER, 101071773). MM declares to have received funding from the European Union (ERC, BIOSFER, 101071773), the Strategic Research Council (SRC), FLUX consortium, decision numbers 345130 and 345131; by the National Institute on Aging (R01AG075208); from any grants to the Max Planck – University of Helsinki Center from the Max Planck Society, Jane and Aatos Erkko Foundation, Faculty of Social Sciences at the University of Helsinki, and Cities of Helsinki, Vantaa and Espoo. The funding sources were not involved in the preparation, analysis, and interpretation of the data or submission of this report., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Study Question: Is maternal pre-pregnancy BMI associated with semen quality, testes volume, and reproductive hormone levels in sons?, Summary Answer: Maternal pre-pregnancy BMI was associated with an altered reproductive hormone profile in young adult sons, characterized by higher levels of oestradiol, LH, and free androgen index (FAI) and lower levels of sex hormone-binding globulin (SHBG) in sons born of mothers with pre-pregnancy overweight and obesity., What Is Known Already: Evidence suggests that maternal pre-pregnancy BMI may influence reproductive health later in life. Only one pilot study has investigated the association between maternal pre-pregnancy BMI and reproductive health outcomes in sons, suggesting that a high BMI was associated with impaired reproductive function in the adult sons., Study Design, Size, Duration: A population-based follow-up study of 1058 young men from the Fetal Programming of Semen Quality (FEPOS) cohort nested within the Danish National Birth Cohort (DNBC), 1998-2019, was carried out., Participants/materials, Setting, Methods: In total, 1058 adult sons (median age 19 years, 2 months), born 1998-2000 by mothers included in the DNBC, participated in FEPOS. At a clinical examination, they provided a semen and blood sample, measured their testes volume, and had height and weight measured. Maternal pre-pregnancy BMI was obtained by self-report in early pregnancy. Semen characteristics, testes volume, and reproductive hormone levels were analysed according to maternal pre-pregnancy BMI categories and as restricted cubic splines using negative binomial and ordinary least square regression models. Mediation analyses examined potential mediation by the sons' birthweight, pubertal timing, fat mass, and BMI. Additional analyses investigated the role of paternal BMI in the potential associations between maternal BMI and reproductive health outcomes., Main Results and the Role of Chance: We found no consistent associations between maternal pre-pregnancy BMI and semen characteristics or testes volume. Sons of mothers with higher pre-pregnancy BMI had higher oestradiol and lower SHBG levels, both in a dose-dependent manner. Sons of mothers with pre-pregnancy obesity (≥30 kg/m2) had higher LH levels and a higher FAI than sons born by mothers with normal pre-pregnancy BMI (18.5-24.9 kg/m2). The mediation analyses suggested that the effect of maternal pre-pregnancy BMI on higher levels of oestrogen, LH, and FAI was partly mediated by the sons' birthweight, in addition to adult fat mass and BMI measured at the clinical examination, whereas most of the effect on lower levels of SHBG was primarily mediated by the sons' own fat mass and BMI. Paternal BMI was not a strong confounder of the associations in this study., Limitations, Reasons for Caution: This study was based in a population-based cohort with a low prevalence of overweight and obesity in both mothers and adult sons. Some men (10%) had blood for reproductive hormone assessment drawn in the evening. While several potential confounding factors were accounted for, this study's inherent risk of residual and unmeasured confounding precludes provision of causal estimates. Therefore, caution should be given when interpreting the causal effect of maternal BMI on sons' reproductive health., Wider Implications of the Findings: Given the widespread occurrence of overweight and obesity among pregnant women, it is imperative to thoroughly examine the potential consequences for reproductive hormone levels in adult sons. The potential effects of maternal pre-pregnancy obesity on sons' reproductive hormone profile may potentially be partly avoided by the prevention of overweight and obesity in the sons., Study Funding/competing Interest(s): The project was funded by the Lundbeck Foundation (R170-2014-855), the Capital Region of Denmark, Medical doctor Sofus Carl Emil Friis and spouse Olga Doris Friis's Grant, Axel Muusfeldt's Foundation (2016-491), AP Møller Foundation (16-37), the Health Foundation, Dagmar Marshall's Fond, Aarhus University, Independent Research Fund Denmark (9039-00128B), and the European Union (ERC, BIOSFER, 101071773). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible. The authors declare that they have no conflict of interest., Trial Registration Number: N/A., (© The Author(s) 2023. Published by Oxford University Press on behalf of European Society of Human Reproduction and Embryology.)

With historically similar patterns of high and stable cohort fertility and high levels of gender equality, the Nordic countries of Sweden, Finland, Norway, Denmark, and Iceland are seen as forerunners in demographic behavior. Furthermore, Nordic fertility trends have strongly influenced fertility theories. However, the period fertility decline that started around 2010 in many countries with relatively high fertility is particularly pronounced in the Nordic countries, raising the question of whether Nordic cohort fertility will also decline and deviate from its historically stable pattern. Using harmonized data across the Nordic countries, we comprehensively describe this period decline and analyze the extent to which it is attributable to tempo or quantum effects. Two key results stand out. First, the decline is mostly attributable to first births but can be observed across all ages from 15 to the mid-30s. This is a reversal from the previous trend in which fertility rates in the early 30s increased relatively steadily in those countries in the period 1980-2010. Second, tempo explains only part of the decline. Forecasts indicate that the average Nordic cohort fertility will decline from 2 children for the 1970 cohort to around 1.8 children for the late 1980s cohorts. Finland diverges from the other countries in terms of its lower expected cohort fertility (below 1.6), and Denmark and Sweden diverge from Finland, Iceland, and Norway in terms of their slower cohort fertility decline. These findings suggest that the conceptualization of the Nordic model of high and stable fertility may need to be revised., (Copyright © 2021 The Authors.)