The purpose of this thesis was to investigate: 1. The effect of lifestyle intervention in women with obesity and infertility on markers of oxidative stress (OS) and associations between markers of OS and cardiometabolic parameters; associations between maternal preconception insulin resistance (IR) and offspring’s birthweight and role of bile acids (BA) in this association. (Chapter 2-3) 2. Differences in diet, exercise, quality of life (QoL) as well as differences in lifestyle change after randomized to lifestyle interventions between women with obesity and infertility with and without polycystic ovary syndrome (PCOS). (Chapter 4-5) 3. The discriminatory value of steroid hormones for PCOS; profiles of biochemical hyperandrogenism and metabolic characteristics in women with infertility and obesity with and without PCOS; and how steroid hormones and anti-müllerian hormone (AMH) changes are associated with resumption of ovulatory cycles in women with PCOS. (Chapter 6-7) 4. The effectiveness of lifestyle intervention prior to infertility treatment on fertility outcomes in women with obesity and infertility. (Chapter 8-12)