Two-sample Mendelian randomization (MR) is increasingly used to strengthen causal inference using observational data. This method allows the use of freely accessible summary association results from genome-wide association studies (GWAS) for a range of traits. Some GWAS studies adjust for heritable covariables in an attempt to estimate direct effects of genetic variants on the trait of interest. One, both or neither of the genetic instrumental variables (IVs)-exposure association or genetic IVs-outcome association may have been adjusted for heritable covariables (referred to as GWAS covariables). However, it is unclear how this may affect two-sample MR analysis. We evaluated this in a simulation study comprising different scenarios that could motivate covariable adjustment in a GWAS. Our results indicate that the impact of covariable adjustment is highly dependent on the underlying causal structure. In the absence of residual confounding between exposure and covariable, between exposure and outcome, and between covariable and outcome, using covariable-adjusted summary associations for two-sample MR may eliminate bias due to horizontal pleiotropy. However, the presence of residual confounding (especially between the covariable and the outcome) leads to bias upon covariable adjustment, even in the absence of horizontal pleiotropy. Bias was lower when the true causal effect of the exposure on the outcome was zero compared to a non-zero causal effect. In an analysis using real data from the Genetic Investigation of ANthropometric Traits (GIANT) consortium and UK Biobank, the direction of the causal effect estimate of waist circumference on blood pressure changed upon adjustment of waist circumference for body mass index. Our findings indicate that using covariable-adjusted summary associations in MR should generally be avoided. When that is not possible, careful consideration of the causal relationships underlying the data (including potentially unmeasured confounders) is required to direct sensitivity analyses and interpret results with appropriate caution. 3

Gestational diabetes mellitus (GDM) is associated with increased risk of pregnancy complications and adverse perinatal outcomes. GDM often reoccurs and is associated with increased risk of subsequent diagnosis of type 2 diabetes (T2D). To improve our understanding of the aetiological factors and molecular processes driving the occurrence of GDM, including the extent to which these overlap with T2D pathophysiology, the GENetics of Diabetes In Pregnancy (GenDIP) Consortium assembled genome-wide association studies (GWAS) of diverse ancestry in a total of 5,485 women with GDM and 347,856 without GDM. Through trans-ancestry meta-analysis, we identified five loci with genome-wide significant association (p<5×10-8) with GDM, mapping to/near MTNR1B (p=4.3×10-54), TCF7L2 (p=4.0×10-16), CDKAL1 (p=1.6×10-14), CDKN2A-CDKN2B (p=4.1×10-9) and HKDC1 (p=2.9×10-8). Multiple lines of evidence pointed to genetic contributions to the shared pathophysiology of GDM and T2D: (i) four of the five GDM loci (not HKDC1) have been previously reported at genome-wide significance for T2D; (ii) significant enrichment for associations with GDM at previously reported T2D loci; (iii) strong genetic correlation between GDM and T2D; and (iv) enrichment of GDM associations mapping to genomic annotations in diabetes-relevant tissues and transcription factor binding sites. Mendelian randomisation analyses demonstrated significant causal association (5% false discovery rate) of higher body mass index on increased GDM risk. Our results provide support for the hypothesis that GDM and T2D are part of the same underlying pathology but that, as exemplified by the HKDC1 locus, there are genetic determinants of GDM that are specific to glucose regulation in pregnancy.

Background Higher birthweight is associated with higher adult BMI. Alleles that predispose to greater adult adiposity might act in fetal life to increase fetal growth and birthweight. Whether there are fetal effects of recently identified adult metabolically favourable adiposity alleles on birthweight is unknown. Aim We aimed to test the effect on birthweight of fetal genetic predisposition to higher metabolically favourable adult adiposity and compare that with the effect of fetal genetic predisposition to higher adult BMI. Methods We used published GWAS data (n = upto 406 063) to estimate fetal effects on birthweight (adjusting for maternal genotype) of alleles known to raise metabolically favourable adult adiposity or BMI. We combined summary data across SNPs with random effects meta-analyses. We performed weighted linear regression of SNP-birthweight effects against SNP-adult adiposity effects to test for a dose-dependent association. Results Fetal genetic predisposition to higher metabolically favourable adult adiposity and higher adult BMI were both associated with higher birthweight (3grams per effect allele (95%CI, 1 to 5) averaged over 14 SNPs; p = 0.002; 0.5grams per effect allele (95%CI, 0 to 1) averaged over 76 SNPs; p = 0.042, respectively). SNPs with greater effects on metabolically favourable adiposity tended to have greater effects on birthweight (R2 = 0.2912, p = 0.027). There was no dose-dependent association for BMI (R2 = -0.0019, p = 0.602). Conclusions Fetal genetic predisposition to both higher adult metabolically favourable adiposity and BMI is associated with birthweight. Fetal effects of metabolically favourable adiposity-raising alleles on birthweight are modestly proportional to their effects on future adiposity, but those of BMI-raising alleles are not.

Anti-Müllerian hormone (AMH) is required for sexual differentiation in the fetus, and in adult females AMH is produced by growing ovarian follicles. Consequently, AMH levels are correlated with ovarian reserve, declining towards menopause when the oocyte pool is exhausted. A previous genome-wide association study identified three genetic variants in and around the AMH gene that explained 25% of variation in AMH levels in adolescent males but did not identify any genetic associations reaching genome-wide significance in adolescent females. To explore the role of genetic variation in determining AMH levels in women of late reproductive age, we carried out a genomewide meta-analysis in 3,344 pre-menopausal women from five cohorts (median age 44-48 years at blood draw). A single genetic variant, rs16991615, previously associated with age at menopause, reached genome-wide significance at P=3.48×10 -10 , with a per allele difference in age-adjusted inverse normal AMH of 0.26 SD (95% CI [0.18,0.34]). We investigated whether genetic determinants of female reproductive lifespan were more generally associated with pre-menopausal AMH levels. Genetically-predicted age at menarche had no robust association but genetically-predicted age at menopause was associated with lower AMH levels by 0.18 SD (95% CI [0.14,0.21]) in age-adjusted inverse normal AMH per one-year earlier age at menopause. Our findings support the hypothesis that AMH is a valid measure of ovarian reserve in pre-menopausal women and suggest that the underlying biology of ovarian reserve results in a causal link between pre-menopausal AMH levels and menopause timing.