The goal of this prospective cohort study is to learn about the overall and seasonal prevalence of vitamin D insufficiency and its determinants in a population of women with infertility who will go through in vitro fertilization in Sweden. The main questions it aims to answer are:

• What is the prevalence of vitamin D insufficiency?
• What are the determinants of this insufficiency? Participants will be asked to complete a questionnaire validated for dietary assessment of vitamin D intake and sun exposure habits and they will leave a blood sample for measurement of 25(OH)D concentration.

Researchers will compare the in vitro fertilization outcomes between women with vitamin D insufficiency and sufficiency to see if there is any difference.

Inclusion Criteria:

• women aged 18-38 years, who will go through in vitro fertilization for infertility.

Exclusion Criteria:

• women who will go through pre-implantation genetic testing
• women who will go through sperm- or oocyte donation treatment
• Fertility preservation cycles

Women aged 18-38 who will go through in vitro fertilization during 2020-2021 (one year) for infertility at the Department of Obstetrics and Gynecology, Sahlgrenska University Hospital in Gothenburg, Sweden.

All IPD that underlie results in a publication will be made available on reasonable request.