Partly because of perceived health benefits and environment concerns, vegetarian/vegan diets have become increasingly popular around the globe in recent years. Despite the possibility of low intakes of some nutrients such as protein, omega-3 fatty acids, vitamin D, vitamin B12, iron, calcium, zinc, and choline, evidence suggests that vegetarians have generally better long-term health than their omnivorous counterparts. Pregnancy is a special period in the lifecourse, when foetal development requires increased nutrient intakes and accelerates nutritional depletion, and adequate and balanced nutrition is critical for both maternal and foetal health. Two reviews have suggested that appropriately planned and well-balanced vegetarian/vegan diets during pregnancy may be considered safe for the women and their fetuses. However, little is known about whether these diets will have longer-term effects on offspring health throughout childhood. Vegetarianism is highly socially and culturally patterned, so there is a need to carefully control for confounding to infer causal effects. This PhD aims to study associations between maternal vegetarian diet and offspring health across several domains and to attempt to infer which associations are likely to be causal. It also aims to study the role of DNA methylation and circulating metabolites as mediators of any effects.